[<< wikibooks] Gmail/The Design
Gmail was created to be extremely easy to use, with as many useful features as possible, while omitting unnecessary options. All the standard e-mail features you'd expect are here, including automatic filters, easy forwarding and replying, and group mail composition. So, what sets Gmail apart from the rest?
Quick Previews
Whether you're looking at your inbox through your customized Google homepage, the Gmail Notifier, or in your inbox, you'll notice that Gmail gives you more than the sender name and the subject. After the subject (in bold), Gmail uses any extra space available to give you a quick preview of the e-mail's content.
Easy Searching
In your inbox, you can use the convenient search box to find any keyword in your inbox and archives. If you don't find what you're looking for, you can expand your search to include spam and trash.
Archives
Do you have an e-mail that you just can't throw away? If you don't want it to clutter your inbox, simply select the checkbox next to the e-mail, and click Archive. The e-mail no longer appears in your inbox, but you can easily access it through the search feature or by choosing All Mail in the left navigation.
Stars
This feature is the equivalent of "flagging". To set an e-mail apart from the rest and include it in a special category, click on the hollow star next to any e-mail. (You can also choose Add Star in the drop-down menu of any e-mail.) To star multiple e-mails at once, check them all, and choose Add Star from the drop-down menu.
Labels
If you'd like to keep all your online shopping receipts together or create a category for e-mails from friends, family members or email lists, you can create a label by clicking on New Label... in the drop-down menu. To apply a label, click on the label you want to apply in the drop-down menu. 
Auto Contact Additions
When someone e-mails you who is not already in your address book, Gmail automatically adds the e-mail address and contact name to your address book. As you type a name or e-mail address into the To: box, Gmail will suggest matching names and e-mail addresses from your current contacts.
Trash
Emails in the trash are automatically deleted forever in 30 days.