Closures and Callbacks 

function sayHiLater(){
    var greeting = 'Hi!';
    setTimeout(function(){
        console.log(greeting);
    },3000);
}
sayHiLater();

Callbacks - A function you give to another function, to be run when the other function is finished 

function tellMeWhenDone(callback){
    var a = 1000; // some work
    var b = 2000; // some work
    callback(); // the 'callback',it runs the function I give it
}

tellMeWhenDone(function(){
    console.log('I am done');
})

tellMeWhenDone(function(){
    alert('I am done');
})