setTimeout is probably the first place you ever used a callback.

gistfile1.js

// Let's make an alert 2s after a page loads.
// We'll need to set a timeout.
// What are the ways we can make Timeouts?

// This was the way I first learned.

function hello() {
  alert("Hello!");
}

setTimeout(hello, 2000);

// See that hello there? That's not a string. That's not the name of the function, it's **the function itself**.
// Why is that possible?
// Well, the above is identical to this:

var hello = function() {
  alert("Hello!");
}

setTimeout(hello, 2000);

// Functions are objects in Javascript, just like strings, arrays, numbers, and so forth.
// All the function keyword does is make a new function object with that name.
// So here you can see more clearly what's going on: we're making a function object called hello, and passing that in.

// Of course, we don't need to pass in a named object.
// We can just make an _anonymous function_ - one with no name, that we define in line. Like so:

setTimeout(function() {
  alert("Hello!");
}, 2000)

// We know that 'function' makes a new function object.
// So that inline, un-named function definition... just evaluates to a function object.

// In all these cases, when 2000ms are up, the first parameter of setTimeout is called.
// In all these cases, that first parameter is a function object.

I wonder how passing data could be demonstrated in this example... maybe:

// Let's redefine the callback to take a parameter

function hello(name) {
   alert("Hello " + name + "!");
}

// Now in the case of setTimeout, you can pass parameters to the callback by adding them like this
// setTimeout(callbackFunction, timeToWait, param1)

// So if we write:

setTimeout(hello, 2000, "Tom");

// Our hello function will receive the name "Tom" and will alert the string "Hello Tom!"