Look it up:
One of the key rules of storytelling is that your characters should change in the course of the story. Normally, a film or a novel will take a couple, maybe a small handful of core characters, and show them being altered by their experiences. If the story is intense enough, you can end up really caring for and empathizing with these characters as if they were close friends. In Crash there are a dozen key characters, all of whom undergo enormous upheavals in their lives. The crises all stem from racism, both understated and overt, and from the distance we keep from each other.
The cast is amazing, and the performances they give uniformly excellent. The script is astonishing, and the direction is calculated to wring the maximum effect from it. Even though there are very few truly terrible things that happen in the film, the constant likelihood and imminence of those terrible things keeps you on a knife edge. The relief when they don't come to pass is like dams bursting. Make no mistake: this film will put you through a wringer. I'm a big softy, so it's not unusual for me to cry at the end of a film. What is unusual is for me to be in tears from the middle of a film onwards, and an emotional wreck for hours afterwards.
Crash is a truly great film. I can't recommend it highly enough.