People who stay in specific areas and wait for targets to come to them.
Though when you have a long-range weapon like a Sniper Rifle, a target 'coming to you' can be from literally across the map; so long as they're in your sight lines. It's why camping with a Sniper Rifle, to people with at least half a brain, is fine. They're not intended to be close-range weapons but Call of Duty's quickscoping fad has warped the role of Sniper Rifles into some "rush" weapon.
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The real camping that's bad is hallway camping. Sitting (or prone. ugh) in a back corner looking down a long hallway or path with only one entry way. If somebody stays stationary at the top of the hill in the middle of an open map that's cool by me; that's why I do and it takes skill to take on and kill people from all directions. But if you're camping the same way people camp with a shotgun only using a sniper and the lone doorway is a fair distance away, that's when you're a dirty camper.