OpenArena supports various game modes, with slightly different rules. Mainly, you have to choose among varous gametypes. You should take a look to Manual/Gamemodes, and to Basic game options for inofs about fraglimit, capturelimit and timelimit. Additional mods may add new gametypes and options. You can find some examples of their use in the Configuration examples page. Note: some of the settings listed in this page are from Quake 3 Arena (ancestor of OpenArena), but many of them have been added by OpenArena itself and thus will have no effect while running old mods.