Digital audio and video files often need to their formats converted for various uses - the web, broadcast, mobile devices, etc all require different formats and codecs to work best. The process of putting one format in and getting the same content out, in a different format, is transcoding.
For example, a project completed as an Uncompressed 8-bit Quicktime file will be too large to stream over the web. To do so, it must be transcoded to a more highly compressed form, such as h264.
Another example, in the digital cinema workflow, would be shooting with a camera that records video in an interframe codec (ie h264). Because of the instabilities of using this type of compression with most current professional editing platforms, the footage will need to be transcoded into an intermediate format, ideally one using intraframe compression.
Transcoding Tools
For single-file conversions, you can use Quicktime Pro-->Export
Batch conversions can be done using Compressor, Adobe Media Encoder, or MPEG Streamclip - among many options out there.
Basic Process
Each program will have specific methods, and the Help menus/ User Guides for all of them are readily available.
1.) Input
Select the file you want to convert. If you are converting a batch, you can select multiple files.
2.) Settings
Determine the desired settings for the finished product.
Codec
Frame size
Frame rate
Audio format/ sample rate/ bit depth
Aspect ratio
Pixel aspect ratio
3.) Output
Determine what the new file(s) will be named, and what folder they will be written to.