It's not strictly a cop show either though. Every season has a main focus; law enforcement (s1), unions (s2), politics (s3), schools (s4), and the media (s5). The show deals with the general incompetence of all these institutions in Baltimore (but it's supposed to apply to any major American city, I guess). Of course, mixed in with this is the everyday street life of those at the bottom of the food chain, and you get to follow them as the perpetual victims of a failing system.
Just an incredible show overall, but sadly it doesn't get the attention it deserves.