Find HTML elements:
-------------------
var x = document.getElementById("main");
var y = x.getElementsByTagName("p");
var x = document.getElementsByClassName("intro");
var x = document.querySelectorAll("p.intro");

Change HTML Elements:
---------------------
document.getElementById("p1").innerHTML = "New text!";
document.getElementById("myImage").src = "landscape.jpg";
document.getElementById("p2").style.color = "blue";

Event handling:
---------------
element.addEventListener("click", myFunction);
function myFunction() {
    alert ("Hello World!");
}