The Ministry of Magic is the governmental authority of the for the wizarding world in Britain. The basic mission of the Ministry is to keep the Muggles from finding out about an entire culture of magical folk. It's a huge, complicated, and inefficient bureaucracy, just like most governmental organizations.
The Ministry consists of seven departments of which the Department of Magical Law Enforcement is the largest. All of the other departments are in some way answerable to it, with the possible exception of the Department of Mysteries. -HPL