The rating system is similar to that used for chess. You gain or lose
points based on the difference between your rating and that of your
opponent. If you're playing someone with a much higher rating than
your own, you'll gain more points if you win, and lose fewer points if
you lose. Against players with a much lower rating, the converse is
true: you gain fewer points for losing and lose more points for
losing.
Multiplayer games are, IIRC, scored as if you had played each player
in a 2-player game and won or lost against that person, but with the
total points won or lost dropped down to reflect that it was only one
game. So in a three player game, where Alice is 1st, Barney is 2nd,
and Chris is 3rd, Barney is scored as if he had lost to Alice in a 2
player game and beat Chris in a 2 player game, but the points involved
are, I dunno, 2/3rds what they would be for two actual two player
games. I think the specific math involved is pretty fiddly.