DISCLAIMER: This is a very late response to this particular post but as I've been reading through various responses to this question, it struck me that most of the answers use terminology that can only be understood by experienced coders. This answer is an attempt to address the original question with a novice audience in mind.
The little '(e)' thing is actually part of broader scope of something in Javascript called an event handling function. Every event handling function receives an event object. For the purpose of this discussion, think of an object as a "thing" that holds a bunch of properties (variables) and methods (functions), much like objects in other languages. The handle, the 'e' inside the little (e) thing, is like a variable that allows you to interact with the object (and I use the term variable VERY loosely).
Consider the following jQuery examples:
$("#someLink").on("click", function(e){ // My preferred method
e.preventDefault();
});
$("#someLink").click(function(e){ // Some use this method too
e.preventDefault();
});
What's happening?
When a user clicks on the element with the id "#someLink" (probably an anchor tag), call an anonymous function, "function(e)", and assign the resulting object to a handler, "e". Now take that handler and call one of its methods, "e.preventDefault()", which should prevent the browser from performing the default action for that element.
Note: The handle can pretty much be named anything you want (i.e. 'function(billybob)'). The 'e' stands for 'event', which seems to be pretty standard for this type of function.
Although 'e.preventDefault()' is probably the most common use of the event handler, the object itself contains many properties and methods that can be accessed via the event handler.
Some really good information on this topic can be found at jQuery's learning site, http://learn.jquery.com. Pay special attention to the Using jQuery Core and Events sections.