Really basic FUNCTION question

Really basic FUNCTION question

I'm looking at a friend's code to learn some things, and i'm confused about a couple of things:


1.
      function fadeSquareIn($i) {

fadeSquareIn is a created function, and he passes in "$i" .....

as far as i can tell, the i variable is not located anywhere else.
the $ sign i thought was to call a jquery event, but in this case, the variable i is not any css element. using firebug, i run through this line, and its as if "i" is just a variable used in the subsequent actions... a for loop, etc...

2.


      $.fn.setImageSource = function(){

what is the .fn for? i looked deeper and saw that its a call to the jquery.js file... i'm not sure what it means or how it is to be used.

would be great if anyone could clarify these basic points.

thanks!