I'm not 100% sure how it works exactly, but I know that everyone starts at 100. Wins increase the rating, losses decrease the rating. (I believe there is more to it than that, such as captures/offensive or defensive kills in control, etc. But I'm not sure exactly what else it takes into account. ) If the enemy team has a much higher average combat rating and you win then your combat rating goes up alot, if you lose it goes down, but not by much. Someone else can correct me of I'm wrong

The destiny app gives each character a combat rating when looking at crucible stats. I'm not sure how it's calculated, but a score of 100 seems to be about average.