The Camp is another one of the "special" improvements, which may only be built on certain resources. Camps tend to provide more Gold income than anything else, which is uncommon for tile improvements, but may turn very important in the early game, especially for cities without 'Plantationable' Luxuries - these will suffer seriously from lack of Gold, and the Camp comes handy to remedy that.
Civilopedia entry Edit
Once upon a time, a camp was vital to survival ... whether it was a temporary shelter thrown up by wandering barbarians or a semi-permanent hunting camp supplying game, fish or fowl to the growing population. Until mankind learned to domesticate and pen livestock for a more dependable source of meat, hunting camps kept civilization going. In time, camps took on new roles, such as supplying furs, firewood, or trading opportunities with the nearby natives. Today, more often than not, a "camp" – temporary or permanent – is simply a recreational site where the city slickers can "get back to nature." And now civilized folk hunt game, fish, and fowl for sport rather than survival.