Javascript 's methods

In every programming language .(As I heard) we need to use pre defined methodes , Key words.
In Javscript 's we need to use perdifined methods and key words as well. When I am doing a tutorial 4 (3SFE407 ) there was a ..... :p

sname=document.forms[0].lastname.value;
In this case :

sname is called as a 1 st form's [indexing (like an array index)] Last Name .that textfield's
name is lastname.
that  "document.forms[0].lastname.value" get in a value to sname.

firstletter=sname.charAt(0);

In this case :
This is a predefined method what we get in to surname's  {This a person's surname} first charachter is pointed and that value
assign to firstletter{it should be a letter}

<div  id="title">
User Registration
div>

In this case:
We are changing the title according to the surname.here we need to get a  id to  the title.its like a variable.
document.getElementById("title").innerHTML=title;
In this case:
Here what we got as the titles name  is refered. then it changes the title name according to your lastname