Difference between JavaScript functions?

Possible duplicate:
what is the difference between the two functions / approaches

I have seen various types of functions posted around, but I cannot find an answer to the differences between them. I know how to write these two, but I don't know the difference.

function aFunction() {

}

var aFunction = function() {

}

      

        
        
        
      

    

Then I heard about things like closures, among other names. I am new to programming, sorry, can anyone please tell me all the different types and the differences between them?