There are lots of different types of mahjong but I play “riichi mahjong” also known as “Japanese rules mahjong.”
I’ve played lots of different versions, but riichi was my first and true love. Riichi mahjong is distinguishable from other rule sets by the following:
-At least one special hand condition is required to “go out” (yaku)
-You cannot win on a discarded tile if you have previously discarded a tile that would have allowed you to win (furiten)
-When you are waiting on only one tile to win, you can call that you are waiting or fishing and increase (riichi)
There are of course many other differences, but these are the big ones.