Where to play Starcraft on the Internet
Battle.Net (www.battle.net): Created by Blizzard, Battle.net is a free way to play Starcraft on the internet. It is available to everyone who has purchased the game. At any time of day, you can find several thousand players on bnet, playing a wide variety of game types and maps. A good service- but it has it's problems. Cheating is fairly widespread, and disconnectors are everywhere.
Kali (www.kali.net): For a one-time $20 fee, you can get Kali, and free upgrades for life. Kali is a program which allows you to play IPX/TCP network games over the internet. Instread of several thousand players, you can typically find several hundred. The people no kali can be very helpful if you're wanting to learn the game. Overall, you can find better games on Kali, because no player records are recorded- so it's all in the name of fun.
The Training Center prefers Kali over bnet. We have found that Kali has far less lag- therefore game speeds run faster. But our primary reason is that we can play on Kali and not worry about the result! It allows you to have far more fun while playing.
Also, many of the all-time SC greats got their stars playing War2 on Kali and still play there. A lot of them are always glad to help players out.