ISA stands for International Space Agency. The ISA are the blue team in game. The ISA represent the forces of Earth and science sent to capture or kill the MMC.
MMC stands for Moon Mining Cooperative. The MMC are the yellow team in game. The MMC represent the forces of the Moon and commerce and are trying to survive after being cut off from Earth.
ZGSE stands for Zero-G Sports Entertainment Inc. The only reference to the ZGSE is on a news bulletin in the map Moondust. Apparently they host games which require a lack of gravity. They've been struggeling financially since their inaugural games four years ago, but have been making a come back since last year when they signed the MMC on as their main sponsor.
Quantamun is a company working on the moon before the explosion which was studying quantum physics. They had full particle accelerator lab: surface based offices, underground curved particle accelerator tube, hazmat suits and the reactor needed for the accelerator. The remains after the explosion can be seen on searchlight.