Global Warfare is a Team Based Multi-Player Half-Life Modification. That means if you have Sierra's Half-Life game then you can download and install Global Warfare and play it!
There are several ways to "win" in the game. The most basic way to win is to destroy the enemies' transport (usually a blackhawk helicopter or an APC) and then kill the remaining troops on the ground. The other ways to win depend on the particular map itself. Some mission objectives are to call in airstrikes, destroy buildings or radio relays or bridges, and others involve capturing/rescuing civilians or officials. See the manual for more info.