The Shield starts Michael Chiklis as Vic Mackey and is about an inner-city Los Angeles police precinct where cops have no shame breaking the law, if they feel it makes the streets safer in the big picture, as well as preserves their own indicidual goals and interests. Mackey is the leader of the Strike Team, which is a four-man anti-gang team with a tight bond. Mackey is definitely the alpha in the show and frequently butts heads with the more "moral" cops in the precint. At first, I hated the guy but as the season wears on, he's begun to grow on me as he appears to view himself as a good guy, even though he frequently appears morally broken.
While Mackey is the star of the show, you also find yourself identifying with the other characters and become intertwined in the other subplots as well. Even though some people may call it blasphemous, from what I've seen, it somewhat reminds me of a faster paced Wire, while focusing almost entirely on the police, rather than focusing on the criminal's story as well.
The show has an excellent storyline, has plenty of action while pushing what's appropriate for cable TV and most surprisingly, Mary occasionally, somewhat tolerates it. If you've never seen the Shield, I suggest looking around the internet for a free Hulu Plus trial and doing your best to cram it in as quickly as you can.