Derek Vineyard (Norton) was a good student with everything going for him until his father, a firefighter, was shot and killed by a black man. Derek was forced to drop out of school to help support his family. He soon started following a white supremacy group in which he later became a leader of.
After being sent to jail for 3 years for a race related murder, Derek is released with a whole new view on life. Upon his release, he comes to find out that his little brother Danny (Furlong) is heading down the same path that he was. He is now hanging out with the same hate group and commits the same crimes.
Derek sits Danny down and explains the horrors of prison. Danny understands where his brother is coming from and vows to stop his doings, and he and Derek were to pick up and move their family and start a new life. This plan ends too abruptly when Danny is shot and killed the very next day in school by a black student he had had a confrontation with the previous day.
Topher
Answer: The tattooed DOC on gang members' arms means Disciples of Christ - the gang that they belong to in the movie.