Also, for those of you that like anime, this is the kind of mahjong they play in all those animes, I think.
Here are the primary differences:
A) No "Chicken" hands (i.e. your hand must be worth at least one point/winning combination)
B) 13 Tiles. Taiwanese Mahjong tends to have 16.
C) Riichi. Riichi is a method to have a winning hand. If your hand is closed (meaning you have not made any chi's (eats) pungs, or open kangs (i.e. you have three of something and someone plays the fourth)) and your hand is waiting on a single tile for victory, you can declare Riichi, which does two things:
i) You put 1000 points on the table.
ii) You must discard every tile that comes except for your winning tile, or if you can make a kang with it that doesn't change your wait.
If you win, you get the Riichi yaku, which gives you one point. If you win on the turn that you declared Riichi, you get Ippatsu, which gives you another point. If you declared Riichi on the opening hand, it is a Double Riichi, which gives you another point.
Winning a Riichi also gives you "ura-dora" which are under doras. Which leads me to:
D) Dora tiles. The fourth to the last tile is flipped over and is the dora indicator. This means that the next tile is the dora(for instance, if the indicator is an 8 of circles, the dora is the 9 of circles.) Each dora that you have gives you a bonus point, assuming you have a hand with at least one winning combination (for instance, your hand cannot win if you have 8 doras but no winning combination). Each kang that is made flips over another tile to turn it into a dora. If somebody wins with a Riichi, the tiles under the current Dora indicators are turned into new dora indicators. For instance, if someone wins, and the dora is the 8 of circles, the tile under the flipped over 7 of circles becomes a new dora. So if it is also a 7 of circles, that means there each 8 of circles counts as two dora.
There are many other differences, which you should probably read here: http://en.wikipedia.org/wiki/Japanese_Mahjong_yaku and here: http://en.wikipedia.org/wiki/Japanese_Mahjong_scoring_rules
Getting on Tenhou:
First thing to do is go to www.tenhou.net/0/?L7447. This is a private server for non-Japanese players.
Then click on the thing that looks like this:
You'll be taken to a screen that looks like this:
Type in an ID name that you want and then click OK. Once that's finished, you'll be taken to a screen that looks like this:
To join a game, click on one of the 12 boxes on the left big box. That will put you in line for a game. The top 8 boxes are 4 player games, and the bottom 4 boxes are 3 player games. I'm not too sure what the difference between the left and right sides are, but the top 4 are one round (han-chan) and the bottom 4 are two rounds (two han-chan). I think the very bottom one is also known for having "red dora."
Post your username in this thread and poke around in case people want to play.