Playing defensively and camping can be two very different things, especially in the context of a shooting game.
In fact, camping in a shooter usually translates into "sniping from a easily defended high-point on the other side of the map"; it's often ludicrously effective, especially for racking up kills (made even worse if the sniping spot also houses the sniper rifle). Should a sniper have a good view of the various respawn locations on the map, killing people immediately after they respawn is a strong possiblility; and yes, it's extremely cheap. Good map design should never allow for such camping spots, where a player could dominate the match easily because they're effectively unapproachable because any opposing players have to be funnelled through a choke-point (or have to cross a huge open area with no cover); the only possible way to get to the camper is tactical suicide. The camper might be having fun (in form of a power-trip); but everyone else isn't, they're just getting frustrated due to their inability to be effective.
Playing defensively tends to translate moreso into "defend the key location or objective", or "staying behind cover as much as possible". Waiting for your opponent to strike and then countering their actions might sound fairly similar to camping, but the latter leaves people with only one real viable option -- to leave themselves open for a easy sniper kill while they try to get in close. Waiting for someone to run around the corner while you have a shotgun is probably a better analogy for someone playing defensively; in games like Halo series, there's always grenades (which everyone has) and there's the range limitation to deal with (though things can get silly in the infection gametype, choke-points of any time ruin that).
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Perhaps the best way to distinguish camping and playing defensively is how they're used.
Camping is choosing an easily defended location, as well as having a weapon which allows the camper exclusive to himself which is then used to dominate the match. A sniper spot where the only way to access it is to run straight through his or her scope is the classic example. It's incredibly cheap, because it's absurdly effective and requires little effort.
Playing defensively is similar, but it doesn't allow the player to dominate the map easily; choice of location seldom matters. It's just a matter of staying alive, not dominating the match.