I have only been using Android for about a week now, so excuse me if I am a bit green about how some bits of it work. Every now and then I accidentally do something on my phone (Samsung Galaxy S3), I don't know what it is, but it causes a half-moon image to appear about half way up the left hand side of the screen. This image sits on top of all other applications. If I touch it, it causes a toolbar of apps to appear on the left hand side of the screen. What is this called? What turns this feature on? And, most importantly of all, how do I turn it off once it has appeared? For reasons I cannot fully justify, I find it really annoying.