The retribution paladin has gone through a lot of reworking in the history of the game. The paladin class has had the most extreme changes, going from Overpowered, to worthless, and back and forth. The retribution focuses on holy and meele damage spells that scale with AP (Attack power). The main stat to get for this spec is to socket Str (Strength).
The protection tree is a bit different from the retribution. Al tho it focuses on meele damage, it's job is to hold threat and to keep the raid alive basically. The main stat for this spec is to gem pure stamina, which is the best stat for a tank.
The holy tree is the complete opposite of the previous specs. They focus on pure int and spell power, picking up haste from gear. They are the "Tank healers" of Wow at the moment. Their main spells include Flash of light, Holy shock, and Holy Light. Their ability to heal a tank and another target for 100% of shared healing between makes them a great asset in every raid environment.