Dexter is a dark comedy/drama that follows the life of the a serial killer/vigilante, who also happens to work as a blood spatter technician in the Miami Police Dept. Furthermore, Dexter is a sociopath incapable of feeling the range of normal human emotions, particularly love. One might wonder how the audience can embrace a protagonist that kills without sympathy, but I assure you he is extremely likable. However, despite the fact that he is indeed a killer incapable of empathy, he does follow a strict code of honor which was instilled by his father.
Throughout the show, Dexter uncovers a range of wicked characters that don't deserve to live, many of whom he methodically disposes of. Along the way, he finds the delicate balance of working in a police department by day, while killing at night.
The rest of the cast features a range of colorful characters. His clumsy, foulmouthed sister, Deb (played by Jennifer Carpenter) also works in the police department in the homicide division. They couldn't be more different, yet they seem to compliment each other so well. Doakes, another dectective on the force is a brutish, intense, man....and the only person who seems to know that there just isn't something right about Dexter. His boss, LaGuerta is constantly (although unsuccessfully) trying to get into his pants. The only character that is somewhat boring is Rita, Dexter's damaged girlfriend (played by Julie Benz). However, she is somewhat limited by a formulaic script.
This show works largely due to the great dialogue, much of which is simply Dexter's inner monologue that serves as a narrative for viewers. It's both smart and thought provoking. Because Dexter is a sociopath, he views the world in a way that allows us to see the things we take for granted. Simply having the ability to love another person is a luxury he doesn't have.
Overall, Dexter is extremely funny, suspenseful, and in a twisted way, a feel good show :)