The "Display density" setting allows you to choose how close together the emails are on the list. Comfortable gives you the most space between emails, while compact makes them closer together, allowing you to see more emails before having to scroll down.
To adjust your display density, click on the gear icon in the upper right corner of your inbox and select your desired density from the menu.
Choosing a theme lets you pick a solid color or image as the background for your inbox. It allows you to have some fun with how you'd like your screen to look and gives you a great way to tell your gmail accounts apart at a quick glance.
To change your theme, click on the gear icon in the upper right corner of your inbox and select "Themes" from the menu. This will bring you to a window that allows you to choose from a variety of themes. If you'd like to upload your own image to use, look for the "My Photos" button in the bottom right corner of the theme selection window.
The "Maximum Page Size" setting allows you to choose how many emails it shows on one page before you have to click the arrows at the top of your inbox to view older messages. You can choose 10, 15, 20, 25, 50, or 100. You can also choose the maximum page size for viewing your contacts.
To adjust the maximum page size, click on the gear icon in the upper right corner of your inbox and selected "Settings" from the menu. Near the top of the page, you'll see the maximum page size section. Click the drop down to adjust the number. Be sure to scroll to the bottom of the page and click "Save Changes" before you return to your inbox.
Snippets are the little message previews of the first line or so of the email that show up after the subject of the email in your inbox view. If you like a cleaner look to your inbox, turning this off can make your inbox look less cluttered.
To turn snippets on or off, click on the gear icon in the upper right corner of your inbox and select "Settings" from the menu. Scroll down near the bottom until you see the "Snippets" section. Click the bubble to turn the snippets view on or off. Be sure to scroll to the bottom of the page and click "Save Changes" before you return to your inbox.
Google tries to predict which messages might be important to you and marks them with a little yellow arrow. I find I'm quite capable of deciding whether or not a message is important all on my own. I also tend to like my inbox as clutter free as possible. Getting rid of these little importance markers looks much cleaner to me, so if you find them as useless as I do, you can remove them.
To turn off the importance markers, click on the little gear icon in the upper right corner of your inbox and select "Settings" from the menu. Across the top, click on "Inbox". On the inbox tab, find the "Importance Markers" section. You can click the bubble for "No markers". Remember to click "Save Changes" at the bottom before returning to your inbox.
You can change the type of inbox you are using. This changes how you view your mail. You can choose between Default, Important first, Unread first, Starred first, and Priority Inbox.
To change your Inbox Type, mouse over "Inbox" in the left-side navigation and click on the little down arrow icon that appears. You will then see the list of Inbox Types and can click on one to make your choice. If you mouse over the inbox type in the list, it will give you a description of each type.