Much of the characters were redesigned to be different from their appearance in the films: Agent K has more wrinkles; Agent J doesn't have a mustache; Agent L is a blonde; and Z is now a clean-shaven but burly, white-haired and baggy-eyed man.
Amblin Entertainment, the production company that made the film, didn't work on the series as Steven Spielberg was working with Dreamworks Television on Invasion America (1998), a relatively similar show.
The trivia item below may give away important plot points.
It is rumoured that the show was only supposed to only last for three seasons, but got extended to four. This can be seen in the third season finale, where a Bug attack occurs and Agent J is thrown out of MiB but returns (coming full circle to Men in Black (1997)). The fourth season was also marked by new characters and an altered opening sequence.