These functions are defined in the ASP.NET AJAX client side library, there for you can use them only if you have included ScriptManager control of the ASP.NET AJAX.
$get - This function is equivelant of "document.getElementById" and "element.getElementById" functions. It points to "Sys.UI.DomElement.getElementById" JavaScript function which is defined as part of the ASP.NET AJAX client side library.
Syntax - var obj = $get(id,element);
element - optional (parent element to search, default is document element)
$find - Used to find component by its id. Generally used for fetching behavior of extender control.
Syntax - var obj = $find(id,parent)
parent - optional (parent element to search)
Reference - http://mattberseth.com/blog/2007/08/the_everuseful_get_and_find_as.html