As Nat said, CAL stands for Cyberathlete Amateur League. There is also the CPL which is the Cyberathlete Professional League. There are also a host of other leagues, TWL, Ironglove to name a few. Most leagues cover multiple games, CAL covers 17 games. most games have different sub-leagues as well; HL2 deathmatch for instance has 1v1, 2v2, and 4v4. alter mentioned cal-o, which the o stands for open, which means anyone can join. the order goes as follows:
CAL-Open
CAL-Intermediate
CAL-Main
CAL-Invite

i might have main and intermediate mixed up.