where is the "back" button?

The back button is located at the top left, just to the left of your address bar, If it is missing click View>Customize Toolbar. You will be given this window:

Just drag and drop the Back/Forward buttons to anywhere you desire them to be (on your toolbar of course)

EE

Depends on which browser and which operating system. In the later versions it's a swipe left to right with two fingers on the trackpad.

Captfred

It should be a 3 finger swipe from left to right on the trackpad. Check System Preferences > Trackpad > make sure the navigate option is check.