I've never actually seen an episode myself but I've heard good things.
The Shield was about an experimental division of the Los Angeles Police Department set up in the fictional Farmington district ("the Farm") of Los Angeles, using a converted church ("the Barn") as their police station, and featuring a group of detectives called the Strike Team who will stop at nothing to bring their version of justice to the streets. Michael Chiklis has top billing with his portrayal of Strike Team leader Vic Mackey. The show has an ensemble cast and usually runs a number of separate story lines through each episode.
Detective Vic Mackey was the leader of the Strike Team, a four-man anti-gang unit based on the Los Angeles Police Department's real-life Rampart Division CRASH unit (Rampart was seriously considered as the series name and was even used in some early promotional ads for the series). The Strike Team uses a variety of illegal and unethical methods to maintain peace on the streets, while making a profit through illegal drug protection schemes and robbery. The Strike Team isn't above planting drugs on and coercing confessions out of gang members or framing them. Attempts to give the team a fifth member have frequently led to near-catastrophe for the group.
The Shield has a variety of subplots, notably David Aceveda's political aspirations and internal confrontation of a sexual assault; Vic Mackey's struggle to cope with a failing marriage; Shane Vendrell's rocky, new marriage; and Julien Lowe's internal conflicts between his belief in the teachings of the Bible and his homosexuality.
Common themes are the citizens' distrust of police, the social impact of drugs and gang warfare, and the conflict between ethics and political expediency. Most characters are portrayed as having both vice and virtue. For example, Vic's loving relationship with his children contrasts with his thuggish approach to police work, although his brutality is generally directed at those who seem well-deserving of such treatment. For example, in Season 2, the Strike Team prepares to rob the "Armenian Money Train," a money laundering operation of the Armenian Mafia. Another episode had Mackey cornering a serial rapist, then letting him be mauled by a police dog before calling the dog off.