Craster's Keep is the grand name given to a small, fortified homestead, located several days' journey beyond the Wall. It is the abode of Craster, a wildling who lives there with his wives and daughters. Craster permits the use of the keep as a redoubt and resupply post by the Night's Watch in return for otherwise leaving him alone. The keep is large enough to host a garrison of men.
In the books
Craster's Keep is not a true keep, but instead a log hall surrounded by an earthen dike, also consisting of a sheepfold, pigsty and midden heap. The hall itself is quite large, large enough to accommodate a few dozen people at any one time. Still, for Night's Watch rangers far out beyond the Wall, its preferable to have a roof over their heads.
- Craster's Keep at A Wiki of Ice and Fire (spoilers from the books)