strange code for me

Hello
I am familiar with C++ or php and OOP but for this code syntax I need some explanation. I cant find
this on php.net.

admin\view\template\amazon\item_links.tpl:
....
$(document).ready(function() { ..... -> call function from class document ?
or
$.ajax({ ...... -> ??????? $ cant be an object or a class ?
or
$('#tableThread1').html(thread1); ..... -> I have no idea

Can anyony help ?
Thanks Peter

This is not PHP code . this is jQuery Code ( a very popular JavaScript library )
link - http://jquery.com/