DROBO anyone?

Ok, I'm in the middle of trying to put together a exteral storage solution at home. I was originally going to go with a two external drives for back-ups and a third HD as a offsite back-up. Someone mentioned the DROBO and I check it out but for some strange reason I can not get my head wrapped around the benefits of using the DROBO. Is it a raid or does it just ghost your data? If one drive fill up and you put in another, what is on each of the individual drives? If I go with the DROBO, how do I go about creating a third back-up to take off-site? If someone has a better grasp on this, can you please give me a break down. Thanks!

~e