As others have said it's basically like you tube, but higher quality.
You can have 1 (or more if you pay for more) DVD at a time, and you can stream one movie at a time from the internet at the same time you have a DVD. If you have two devices, generally only one can stream a time. You can stop a movie and change to a different one, but you can't play two at a time (ie: two windows open) as it keeps count and only lets each account stream one at a time.