Sum Serum is an independent and unauthorized browser implementation of the very nice board game Sim Serim by Heinrich Glumpler. You can find English rules on BoardGameGeek.
This implementation lets you play locally, with two players, or online:
If you play locally, you probably do not want to select your number of stones with the mouse, as your opponent will see your selection. Therefore, the left player (black) can use the keys 1 up to 4 to make a (hopefully unseen) selection. The right player uses U, I, O and P, which just happen to be convenient keys on the right. It does not matter in which order the players make their choice.
If you want to play online, just press “Play online”. You will see an URL shown below that button; share that with your friend (e.g. via e-mail or instant messenger) and you can play. After one round you have to start a fresh game and share the URL again.