Final Fantasy VII question?

Can anyone explain to me IN SIMPLIFIED FORM what (i) Before Crisis (ii) Crisis Core (iii) Final Fantasy VII and Dirge of Cerberus is about. Like the story, and how they are connected. You may notice that I left out Final Fantasy VII Advent Children, but that is because I already watched it and understand it (partially) I thought that if I found out more about the series (Crisis Core in particular) I might understand it more. I tried sites like Final Fantasy wikia and others but they seemed confusing. Please keep in mind that if possible, please add spoilers if there are any because I wouldn't be able to play the game anyways...behehehe...Ummm so thanks in advanced.