I've ran about 15 rounds or so in season 3 of competitive, man, that stuff is intense.
Each team gets a turn to attack and defend a point, and the attacking team gets points depending on how far in the map they get. If it's tied, there's a tiebreaker round with a shorter time limit to determine winner. If points are still tied after the tiebreaker round, it ends in a draw.
You cannot make progress without working as a coordinated group, even in a pick-up-group. I've been primarily playing mercy, both because I think I'm most effective as healer currently, and most people seem to prefer tanking or DPS.
the end of map choke points get ridiculously frantic and crowded as you try and push the payload to the last goal.
It's almost a grind, but so satisfying when you end up with a win (or even a draw, if it was hard fought).