Edit:
Wait, I actually found some content that isn't up to date:
In the Options drop-down menu you can choose to mark all messages as read. Important messages or those that otherwise need your attention can be flagged by clicking the flag icon next to the message. Clicking the "Flagged" link on the left shows any messages you have flagged.
-> there is now no "Options" menu on the left side; rather, there is a "Mark All Read" button on the bottom of the page. Likewise, the "Flagged" link is now called "Bookmarks".
