To understand this movie, it helps to know a bit about Detroit. I've been there a lot over the years. Back in the Nineties (just before this film was made, in 2002), the core of Detroit was a burned out husk, and anything south of 8 Mile Road was a wasteland of abandoned buildings and horribly run-down neighbourhoods. The core - right across the river from Windsor, Ontario - has since gentrified to a certain extent, but there's still this massive donut of space from one mile from the river all the way to eight miles out where sane people simply don't go at night. And there's very little reason to go there during the day - unless you're a photographer: there are some gorgeous hundred-year-old civic buildings that were abandoned 50 years ago. According to Wikipedia's entry on the film, 8 Mile Road is also the divider between the poorer black neighbourhoods and the richer white neighbourhoods to the north.
Detroit was (may still be, I have no idea) a hot-bed of rap, and Eminem came up through their vicious system of rap battles, a white guy in the midst of an almost entirely black crowd. In this movie, he plays a slightly different version of himself - just split up from his girlfriend, back living in a trailer with his impressively trashy Mom (Kim Basinger) and four year old sister, working at a metal stamping shop and trying to make it as a rapper with his friends. He has the talent but not the confidence, and one of the first scenes shows him on stage with the mic ... unable to speak. We get a good look at his friends, his family, and his life, all leading up to a climactic final scene back on stage.
Eminem turns out to be a decent actor, at least when he's playing himself - I don't think that's as easy as it sounds. The supporting actors are also good, and the writing is excellent. The movie also brought us what I think is Eminem's single best song, "Lose Yourself," which is effectively a rap retelling of the life of his character. A very good movie.