Warning: Wikipedia contains spoilers
The movie speaks of an organization which calls itself Men in Black. This organisation acts as the Immigration Bureau to planet Earth - it allows aliens onto the planet and monitors them while here. The movie follows an NYPD officer (Will Smith) as he becomes a part of MiB, and his first major mission. Tommy Lee Jones plays 'K', a senior MiB agent who guides 'J' (the code-name assigned to Smith) as he learns the ropes.
A Bug (an alien species, similar in many ways to a very large cockroach) is trying to steal a galaxy from another alien - whose government is willing to have the earth destroyed rather than let this galaxy fall into the hands of the Bugs.
An animated television series followed, based closely on the style of the movie.
The movie sequel Men in Black II or (M.II.B.) was released in summer 2002. (movie's website).
See also: Men in Black