Wow, that's a pretty broad question, I'm not sure I have time to write the book on it.
Basically, I'm not sure that there is a "tournament" format. Koreans are (like most asians) notorious gamblers, and Sa-Gu is just another means of wagering. I guess I'll start with the basics. In Korean, "Goo" means "ball." So Soccer is "Chook-Goo," literally, kicked ball. To apply it to the cueing sports... Billiards is "Dan-Goo," 4 ball is "Sa-Goo" (Literally, 4 balls), straight rail is "Sam-Goo" (Literally 3 balls), 3 cushion is "Tree Cu-shion," and pocket billiards is called "Pocket-bol."
The basic rule format is that you have 4 balls, 2 cue balls and 2 red balls. One of the cue balls is distinguished from the other with a dot. Korean 4 ball-balls are larger and heavier than traditional carom balls. The break is similar to a billiard break shot, except the 2nd cue ball is located directly behind the 1st red ball to be contacted, at the 1st diamond across.
Scoring is like straight rail, except that if your cue ball contacts your opponent's cue ball for any reason, you lose your turn. If you fail to contact either red ball, you lose a point as well. Once you have reached the designated number of points to win, you must "finalize" the game with a 3 rail billiard.
The complexity of the game really shows itself by having the extra "obstacle" on the table in the 4th ball. With a smaller table and larger balls, plus very, very fast cloth, you'd be suprised how much a pain having 1 more ball on the table is.
It's a great game, I love it. I haven't read anything through books or other materials, this is just how the game was explained to me by other Koreans. I could be way off of what other people play, but at least this is the way they seem to play in Houston, Dallas, LA and Seoul.
If you have more questions, I'd be happy to answer, or find out for you next time I go to a Dang-ku-jang (billiard room).