var yourCallback = function() {
    alert('2');
}

function1(params, yourCallback);

function function1(args, callback) {
    //Some code
    if (callback) {
        callback();
    }
}

function doSomething(arg1, callback){
    // bla bla bla
    callback();
}

function one(x, y, z, callback) {
   // do operations here

   callback();
}

one(1, 2, 3, function(){alert('hello');});

var func = function() { alert('hello'); };
one(1, 2, 3, func);

function func() {
  alert('hello');
}

one(1, 2, 3, func);

function myFunctionNeedsACallback(param1, callback) {
    //do stuff with param1
    if(typeof callback === "function"){
        callback.call(yourThisParam, anyOtherParams);
    }
}

myFunctionNeedsACallback(param, function() { alert('2'); });

//will alert 2 after running the rest of the contents of the function

function ajaxCall1(){
    return $.ajax(/*your parameters NO SUCCESS CALLBACK NEEDED*/);
}

function ajaxCall2(){
    return $.ajax(/*your parameters NO SUCCESS CALLBACK NEEDED*/);
}

function ajaxCall3(){
    return $.ajax(/*your parameters NO SUCCESS CALLBACK NEEDED*/);
}

$.when(ajaxCall1(), ajaxCall2(), ajaxCall3()).then(function(results1, results2, results3) {
    //do stuff with all 3 results without "nesting" things
});