Consider your hard drive to be a filing cabinet. Each partition on it as separate drawer in that filing cabinet. In each drawer of the cabinet you could store a number of folders - Gas bills, pay advices, photographs, christmas card list etc. so that when you wanted to look at a particular item you didn't have to rummage through all the other stuff first. You could also subdivide your gas bills, pay advices, photos etc still further into years or topics and put each category into a new folder for easier and quicker reference.
Does that help?