Q:

What does function() mean in jQuery? What is the difference between this and $(this)

Question 1
I've just started learning jQuery and one thing that puzzles me is function() with a parameter in it, ie: function(e).
[Example 1]
 $("#rating a").click(function(e){
     // stop normal link click
     e.preventDefault();
 }

What is e in function(e)? 
[Example 2]
$.post("test.php", function(data) {
    alert("Data Loaded: " + data);
});

How do you know what data is? I assume data is different from e in example 1.
I'm confused as to why function() is different in different senarios...

Question 2
Since we are talking about basic jQuery, here's another jQuery question. What is the difference between this and $(this)? 
[Example 3]
$(document).ready(function() {
 // use this to reset several forms at once
$("#reset").click(function() {
    $("form").each(function() {
        this.reset();
    });
});

[Example 4]
   $("a").hover(function(){
       $(this).parents("p").addClass("highlight");
   },function(){
       $(this).parents("p").removeClass("highlight");
   });

});

A:

function(e) -> this is passing in the event object as a parameter.  Without e you just don't have access to the event object.
"this" is the object in context to whatever you're doing. If you wrap "this" like $(this) then you have access to all the extensions that jQuery gives you. To be more clear, "this" is usually a DOM object, $(this) is a jQuery object.

