Arca Swiss is just a standard for a rail and clamp system that is designed for quick on and off, like a picatinny rail; just faster, a bit larger, and not as repeatable in position.
Ok, Jeremy beat me to the punch. Almost every head is capable of changing out the clamp, so if you have a head that is designed for the Manfrotto style plates you can just get a new clamp rather than replacing the whole head. Arca plates are made by lots of brands and come in lots of lengths, so you can take your pick their.
Most all Arca Swiss stuff plays nice with other Arca Swiss components, though there are a few products that don't match up well as the standard is not as clearly defined as that of a picatinny rail. Personally I have not ran into the problem, just avoided some of the cheaper stuff due to reviews that complain about it.