How can you tell if you have a crop sensor on your camera?

i own a nikon D90. i was watching some viedos and the people are talking about crop sensors and how it can change numbers because you have to times by something or rather. ARGH! anyways i was wondering if anyone can explain to me, how can you tell if you have a ful frame sensor or a crop frame sensor, and which one is better and why would you want to go for a full or a crop.