first cousin once removed? second cousin? I'm confused!

SO, I never really understood the once removed, twice removed thing. FB has a family application, and it has a button to "define relationship" after you have confirmed a family tie. I would really like to make a family tree. I would need some assistance from someone on either side of the fams, but I'd like to do it. Sure wish Grandma's had email. That would greatly help. I'd also like to know, to preserve it for posterity. I come from a long and colorful background, that I don't know as well as I should. Let's see how well I rectify that, shall we?