Photoshop .PSD
Of all the file formats that Photoshop supports, the PSD format is probably the most important. PSD stands for "Photoshop Document", and as the name implies, it's Photoshop's native file format. PSD is one of the few file types that fully support all of the powerful features that Photoshop gives us, like layers, layer masks, adjustment layers, channels, paths, and so on. It also serves as your working file. When we open an image in Photoshop, regardless of which file type the image was originally using, Photoshop temporarily converts it into a PSD file behind the scenes so we can work on it with all of Photoshop's tools, commands and features at our disposal.
PSD files are your best choice to serve as your master files and for archiving to CD, DVD or an external hard drive for safe keeping. If you've done any sort of editing work on an image and there's even the slightest chance you'll need to come back to it again at some point in the future, save your work as a Photoshop PSD file. There's no loss in image quality no matter how many times you re-open and re-save a PSD, and all of your layers, layer masks, adjustment layers and so on will be saved as part of the file, allowing you to go back at any time and make changes to the image or continue working from where you left off.
You can easily print your images at home with Photoshop directly from the PSD file, and many commercial printers are now able to accept PSDs as well, although some may still require an EPS or TIFF version of the file instead, so it's always best to check with your printer to make sure you're giving them the format they need. One of the newer advantages with PSD files is that they can now be imported directly into Adobe InDesign, giving you complete access to the individual layers in the file as you're designing your page layouts. You can even re-open a PSD file in Photoshop directly from InDesign, make changes to the file, save it, and have the changes immediately update in your layout!
The only real disadvantage to PSD files is that the file size can get very large, especially if you're working on an image with hundreds or even thousands of layers. But with computer hard drives and memory being so cheap these days, it's a small price to pay for the creative freedom that Photoshop and its native PSD file format give us. Bottom line, your PSD file is the most important file you can have, so be sure to save yourself a master copy of your work as a PSD file so you can always return to it in Photoshop when you need it!
JPEG
The JPEG (Joint Photographic Expert Group) format has been around for nearly 20 years now and has become the most popular and widely used file format for viewing and sharing digital photos. It supports 24-bit color, which means it can reproduce roughly 16.7 million colors, and even the cheapest digital cameras can capture images as JPEG files. Most high end digital SLR cameras give you the option of capturing images in either the JPEG or RAW format.
It's important, though, not to confuse "popular" and "widely used" with "professional quality". JPEG is what's called a lossy file format because it compresses the images, which essentially means it takes some of your image information and tosses it out the virtual window, never to be seen again. It does this to reduce file size, but the more compression you use, the worse your images look. You control the amount of compression being applied to the file using the Quality setting that appears in Photoshop when you go to save it. A high enough Quality setting can still produce great looking images but your file size will be larger. Lower Quality settings can produce very small file sizes, but set too low and you'll introduce ugly and obvious compression artifacts.
The biggest strength of JPEG files is convenience. They're usually small enough that they can easily be uploaded and displayed on web pages, or on photo sharing sites like Facebook and Flickr, and emailed to family and friends. Online printing services usually require your photos to be uploaded as JPEG files. The downside to JPEGs is that the reduced quality caused by image compression means they're not a good choice for printing when image quality is your primary concern, and they're also not a good choice for archiving your originals.
If you're capturing JPEG files in your camera, make sure you're capturing the largest, highest quality images possible. Check your camera's instruction manual to find out where the image quality option is in your camera's menu system. The highest quality setting is usually labelled "Large".
One thing you want to avoid doing whenever possible is re-saving JPEG files repeatedly. Each time to open and re-save it, you'll add even more compression to the image, and it doesn't take long for things to get ugly. Once the image detail is gone, you can never get it back (unless of course you read the first part of this article and saved a master copy of the original as a Photoshop PSD file).
GIF
The GIF file format, which stands for Graphics Interchange Format, has been around even longer than JPEG, and it's the format of choice for web graphics. Notice I said web graphics, not web photos. GIF files can only display up to 256 colors, far less than the thousands of colors needed to convincingly reproduce a photographic image (and far less still than the millions of colors supported by the JPEG format).
When it comes to web design, though, the GIF format is indispensable. The files are well suited for web page layouts, banners and buttons, especially if they contain large areas of solid color. All major web browsers support GIF files and their small file sizes load quickly on the screen. GIF also allows web designers to create simple animations. One major advantage GIF has over the JPEG format, and another reason why it's so important for web designers, is that it supports transparency, although it supports only one level of transparency, meaning a pixel is either transparent or it's not. This can result in harsh edges around graphics if the edge color differs from the color of the background it's placed over. For higher quality transparency effects, a better choice is the PNG format.
PNG
PNG (Portable Network Graphics) was originally meant to replace the GIF format (PNG also stands for "PNG not GIF"). That never happened and GIF files are still in wide use today, yet the PNG format improves upon the GIF format in nearly every way. It even improves on the JPEG format. While JPEG files support 24-bit color (16.7 million colors), PNG files support up to 48-bit color, giving us more than 1 billion possible colors! That may sound impressive, but even JPEG files support more colors than the human eye can see, so any real world differences between 24 and 48-bit color are minimal at best.
The biggest advantage over JPEG is that PNG is a lossless file format, meaning that even though it still compresses images to reduce file size, the compression method it uses does not result in a loss of image quality. You can even re-save the same PNG file multiple times without degrading its quality, whereas JPEG files look worse each time you re-save them. With over a billion possible colors and lossless compression, PNG is a great choice for saving digital photos as high quality originals. The downside, though, is that PNG is not as widely supported as the JPEG format, and PNG does not support CMYK color, which means commercial printers can't use them. For everyday viewing and sharing of your digital photos, the JPEG format is still more useful and convenient, even if the image quality isn't as good.
PNG's main advantage over GIF files, besides far exceeding GIF's 256 color limit, is that it can reproduce a full 256 levels of transparency compared with GIF's single level, giving us smooth transitions around edges without having to worry about matching the edges with the background color. PNG files are also usually smaller than GIF files, so they'll load even faster in a web browser. Unfortunately, older web browsers may not support the PNG format, which means GIF is still the safest choice when browser compatibility is your main concern. Also, while GIF supports animations, PNG does not. PNG files are most often used in multimedia programs like Flash as well as Keynote and PowerPoint presentations.
TIFF
Like PSD files, TIFF (Tagged Image File Format) is one of the few file types that support all of Photoshop's features and is another great choice for archiving your images, with lossless compression that allows you to save photos with the highest possible image quality. The quality comes at a price though, as TIFF files can be very large, especially when compared with JPEG files. TIFF is the universally accepted standard for images destined for commercial printing and is compatible with virtually all page layout programs like QuarkXPress and InDesign.
Even though TIFF files are capable of storing all of the layers, adjustment layers and other elements you've added in Photoshop, it's generally recommended that you save all those elements in your master PSD file, then use the TIFF format to save a flattened version of the image for print. This makes it easy to tell just from looking at the file extension which version of your image is the master working file (.psd) and which is the flattened, print-ready version (.tif). Also, many commercial printers will ask for a flattened version of your TIFF file.
With InDesign now being able to import and work directly with layered Photoshop PSD files, and both the PSD and PDF formats gaining popularity in the print community, TIFF isn't quite as important as it once was, but it remains the print industry standard and enjoys widespread support.
EPS
EPS (Encapsulated PostScript) is another print industry standard format that's been around for quite a while, but its use has been in decline over the years. EPS files are not really image files in the traditional sense. Instead, they contain a series of instructions for how a printer should reproduce the image. They can be imported into most page layout programs, but the "encapsulated" part means the files are essentially locked and can no longer be modified unless they're re-opened in Photoshop. A preview image must be embedded in the EPS file when you save it in Photoshop if you want it to be viewable onscreen when working in your page layout program, otherwise you won't be able to see it until the layout is printed. While EPS remains an industry standard format, you probably won't use it very often unless it's the format specifically requested by your commercial printer.
Finally, while most people are familiar with PDF files for viewing, sharing and printing electronic documents (hence the name Portable Document Format), PDF is also gaining in popularity as a great choice for saving images destined for print. Like the PSD and TIFF formats, PDF supports and preserves all of Photoshop's features, including the ability to use spot colors, something the EPS format does not support. PDF gives you the choice of either JPEG compression, complete with a Quality setting to balance image quality with file size, or lossless ZIP compression. And the PDF format benefits from the fact that anyone with the free Adobe Reader installed on their computer can view the image.
The most important thing to remember is to save your working Photoshop file as an unflattened PSD file to use as your master copy, which will preserve all of your layers, channels and so on in Photoshop's native file format, allowing you to return to your work at any time. From there, you can save a copy of your image in one of the other six formats depending on where the image is headed (print, the web, or a multimedia program) or which format your printer has requested. And there we have it!