Mass Effect games are RPG-Shooters. They are also fairly linear games, because they are more about telling a story. You get full customization and everything you'd expect from an RPG with one glaring detail omitted - no inventory in Mass Effect 2.
Honestly, if you like Oblivion and games like that, then you should like Mass Effect. If you've played KOTOR, even games like Gears of War, then you will probably like Mass Effect.
I've played all of the Mass Effect games, and while I recommend playing the first yourself, you don't have to if you get the PS3 version. The PS3 version is the superior version of the game, because you get ALL DLC (up until the point of it's release) included, and a nice interactive story that allows you to pick all the important things that happened in Mass Effect 1.