Well, they have several reasons for being on Pandora. The RDA (Resources Development Administration) has set up a colony on the moon mainly to mine a highly valuable mineral known as unobtanium. Some of the humans, however, are there for the science. They are there for different jobs concerning the Avatar program, a science program that consists of bodies that look like the moon's natives, the Na'vi, but have the mind and soul of the body's avatar driver. There are several different jobs in the program, however, such as being a pilot, being part of the security team, Sec-Ops, driving an avatar body, or working in the lab.