This is ususally the button located at the top left of the screen for a very simple reason - it is the single most frequently used in a web browser. It allows you to go back to the page you were on before in case you realize that you clicked on the wrong link and ended up somewhere that doesn't have what you want. Often, it is also the only means available of going back to the page that you were just on.
In most web browsers, the Back button is an arrow of some kind that points to the left. Here is what the button looks like in Internet Explorer, for example:

You may notice the little downwards-pointing triangle to the right of the button. Clicking on this will cause a list of sites that you have visited recently to pop up. Clicking on one of the items in that list will take you straight to that site. This can be useful if you want to go back three or four steps instead of just one.

You can try out the Back button now if you wish by clicking it. It will take you back to the page you were looking at before, in this case, the page that introduces the five basic buttons in a web browser. To move on to the next topic, click on the "Forward to" link at the bottom right of this page.

<<< Back to "Browser Basics" Forward to "Forward Button" >>>