Best ball can be played using 2-, 3- or 4-person teams. Each player on the team plays his or her own golf ball throughout the round, and on each hole the low score - or "best ball" - of the group serves as the team score. Player A gets a 5, B gets a 4, C gets a 6, D gets a 6, then the team score for that hole is 4, because the low score of the group was B's 4.
Best ball is usually played as stroke play with the total score added up at the end of the round. It can be played as match play, but best-ball match play with more than 2-person teams results in a lot of halved holes.
When using 3- or 4-person teams, it's almost imperitave to apply handicaps so that the weaker players will be able to contribute.
A 2-person best ball match play competition is also known as Four Ball.
"Best ball" can also refer to a competition in which a single player plays match play against a 2- or 3-person team playing best ball. That variation is good for a low-handicapper taking on a team of higher handicappers.
Handicap allowances for Best Ball competitions can be found in the USGA Handicap Manual, Section 9-4 (www.usga.com).