A callback is a function you want to have called when something else happens.

A simple example is an onClick function. When you do

element.onClick = myFunction;

You are using callbacks. More specifically, myFunction will be 'called back to' when the element is clicked.

Another example is when doing async calls like AJAX. Because triggering the AJAX call doesn't usually stop the program until it finishes, just moving onto the next thing immediately after making the call could screw things up if it doesn't complete fast enough. To deal with this, we provide a function to call when the request completes, this function is a callback.

Any time you've done something like someCall(a, b, function() {... you've used a callback. window.setInterval comes to mind here.

I hope this clears things up a bit, I remember trying to wrap my head around callbacks way back when. The key was to realize that even though they are used for many advanced and often convoluted things, what they actually are is relatively straightforward.