It's an excellent TV programme, definitely amongst the very best I've seen. It's very dark and dramatic, with top-notch acting and writing. It won 10 Emmy Awards including several for Best Actor for Bryan Cranston. Here's a summary from TV.com (mild spoilers!):
Bryan Cranston (Malcolm in the Middle) stars in this drama focused on a mid-life crisis gone bad for an underachieving high school chemistry teacher who becomes a drug dealer after discovering he has lung cancer. With a new sense of fearlessness based on his medical prognosis, and a desire to gain financial security for his family, Walter White joins forces with an old student, Jesse Pinkman (Aaron Paul), in a quest that follows their entry into a dangerous world of drugs and crime.
I definitely recommend it