A Camper is a player who stays in one place, waiting for players to approach so that the camper may kill them. Camper is a term often used to offend somebody by saying that they can't get kills "normally" and have to rely on "camping" to get some kills. The most effective way to counter camping is to use explosives like grenades or to just go a different way to your destination, if possible.
Despite camping being an infamous tactic, it can still be used to a player's advantage at some times. For example, in the map Junk Flea, it's common for campers to put themselves by the fence by the large truck, enabling them to easily kill the opposition coming from Alpha's spawn. Camping is also very popular in large maps such as Rattlesnake, where you can camp almost anywhere, and have access to the opposing team's spawning points.
Snipers are also usually campers, although experienced snipers move along with an assault rifle, therefore excelling at both close ranged quarters and long ranged combat.