Slim is the soft-spoken, hardworking ranch hand who wins the respect of the other workers. The other men gravitate around him because he is a natural leader. Slim also represents the typical migrant worker of the time - celibate, rootless, nomadic by necessity - who literally cannot afford to settle down. During the Great Depression, there were many such men roaming the countryside looking for farm work, living in precarity.
Crooks, on the other hand, is the outsider - because of his handicap but also because of his colour. After his injury he is delegated kitchen jobs he can handle with one arm. He also suffers from a bad back and is bent over from sclerosis. His name, or rather nickname, comes from the word 'crooked,' meaning 'bent.' For Crooks, this is a humiliation. He keeps to himself, reads a lot, but has grown bitter over time. Crooks is a mean, pathetic old man who nevertheless solicits the reader's sympathy. As Slim, he represents the alienated man, but in a different kind of way.
Fist-swinging Curley is the little mean guy, complexed over his size and always on the lookout for an occasion to prove himself. He is even more antipathetic than Crooks because he isn't really a victim of circumstance. He is the boss's son, a spoiled brat who has never grown up.
Curley's wife is another estranged person and deserves mentioning. She has no identity, not even a first name. For this reason she is perhaps the most isolated character of them all.