A shower on a single RCD is quite common....... Or maybe an RCD would be used before the plug on something like a lawn mower. So if the mower were to cut the flex the RCD would trip the mains out before the user was hurt..........
Showers and sockets, normally but generally anything that's associated with water, portable appliances and any external device(garden lights, mower etc). But saying that external devices should be protected by earth leakage breakers not RCD's
It depends on the system your electricity suppliers have installed. The two common types are, PME (protective multiple earth), and TT (terra ?). If the system is TT then everything goes through the trip, if PME then read mac309's answer. It won't do any harm putting everything through anyway.