Social is where you can play against anyone in PvP. Ranked playlist is where you get a rank based on win/loss (in most games). That way it would be semi competitive and a rank 5 would not play against a rank 50. In Halo, you would be anywhere from rank 1 to 50. The more you win, the higher your rank. If you're a rank 35, you would play against other rank 35 players, or somewhere close to that rank. If you won, you could go up in rank and the losing team could go down. Once you got to rank 50, you would play against top ranked players. In Halo they had other factors like EXP that went into how fast you ranked up after winning a game. That is another discussion on it's own.