What is the purpose of the 3rd on-screen button on a button-less phone? I haven't used Ice Cream Sandwich yet and as far as I know, some phones (Google Nexus perhaps) don't have capacitive or hard buttons. Now as you can see below, there's 4 buttons (this was an image I found while googling).



As far as I know, the buttons are (starting from the left):


Back button
Home button
???
Menu button


I want to know what the third one is? The one that has overlapping rectangles? My phone has capacitive and I have all the above aside from that one button. I asked this just out of curiosity.
That's the "Recent Apps" button. It's alternative of long-press physical home button.

If you tap this, a scrollable screen appears containing thumbnails of suspended or closed recent apps. You can tap thumbnails to switch to apps or swipe it to discard.