The code is inserted into the active workbook. Basically, a Class is the definition of an Object. And object is anything that you want to design. For example, a Worksheet is an object. And there is a class module which defines just what a Worksheet really is.
Properties simply define and set various attributes.
An object also has Methods. For example, a Worksheet object has an Activate method. This causes something to happen. Finally there are Events. I can't think of a good grammatical analogy for events.