Rash vest, rash top, rash guard, rashie: they are all the same thing. A rash vest stops the rash you get from lying on a waxed surfboard, and it protects you from the sun.
Many of the rash vests here are rated UPF 50+, and we recommend one. Even on overcast days it is easy to burn on the water.
Rip Curl makes looser T-shirt style rash vests and more fitted shapes. It comes down to personal preference.
A short sleeve rash vest keeps your shoulders covered. A long sleeve rash vest gives more protection, so there is one less thing to think about while you are catching waves.
For extra warmth, look at Rip Curl's thin neoprene and polypro tops. They are practical for swimming and for watersports in warmer water.