The tabs are there in case you want to look somewhere else in your mailbox while you're reading or composing an email message. They allow you to refer to something in, say, your Inbox, or your "Sent" mail, or even your "Trash"…and then return to the open message you were reading or composing.

In old mail, you couldn't do this. If you clicked back to your Inbox before completing a message, you'd lose what you were working on. And how frustrating was that?

Try out the fab tab function while composing a message. Start by clicking once on your Inbox, to open the basic Inbox view. Then:

Click the Compose button to open a Compose window. Notice the tab along the top, which says "Compose." There's a little pencil icon next to it. Cute! To the left of that tab, check out the "Inbox" tab. See it?

Click the "Inbox" tab to return to your Inbox. From there, you can open messages in your Inbox. You could also do this with messages in other folders—like those saved in "Sent" or "Draft." This is convenient for seeing what you already said to somebody, for example, or for reminding yourself exactly what someone else wrote to you. When you've learned what you wanted to, click the Compose tab again, and you can pick up where you left off. Nice, huh?

This is a "desktop environment." The tabs are supposed to be like sheets of paper you can shuffle through and compare. Awesome!