* In the camp: Physically inside a specific camp. * At the camp: Near or around the camp’s area. * In camp: Actively participating in camp activities (common in the U.S.). * At camp: Also means participating in camp life but used more widely. In short: * "In/at the camp" = location. * "In/at camp" = involvement in camp activities.

Subtle differences only. They usually mean the same thing. It depends on the exact sentence & context.