The TV series draws its name from the first novel in the series and focuses on Uhtred, a young English nobleman's son who is captured in a Viking raid and raised amongst them. Years later, Uhtred returns home and has to deal with his dual heritage and torn loyalties. In the process, he meets and befriends the young King Alfred of Wessex, who dreams of unifying all of England under one king and one god.
There are eight novels in the Saxon Chronicles (so far), giving the BBC a lot of material to draw on if this first season is a success. The first season will debut on 10 October in the United States on BBC America, and will probably air a few weeks earlier in the UK.
The BBC America website has some additional character profiles and background info on the series.