Steinbeck turns the conventional buddy system on its ear by having one of the two good friends, Lennie, be mentally impaired. Despite his disability, George, the more "with it" of the two friends, protects Lennie despite the costs to his livelihood and his status among his peers.
The story is a fable of good vs. evil, of friendship vs. expolitation. One of the primary characters, Curley's wife (never given a name) struggles with issues of dependancy and identity, a factor that is prevalent in all of the social situations of the novel, whether the relationship is between George and Lennie, Candy vs. Slim, or the "real" world vs. the idealized world to which George and Lennie aspire.
Steinbeck also examines the social system that puts characters who are uneducated and down-on-their luck against a system which seems to be designed to keep them down. All in all, Of Mice and Men is an examination of the social constructs that existed in the early twentieth century and in some ways, still exists today.
Symbols: (only some)
1. In the beginning of the book, everything is peaceful until George and Lennie come in, arguing. Also, a karp jumps up out the water (to eat somthing, I believe) and creates a ripple. Similarly, Lennie makes ripples with his finger and says "Look what I done George". The ripples then come back to the center. This symbolizes the ripple effect of Lennie's actions. He starts with mice, then kills a puppy, and eventually kills Curley's wife. The effects reach back to him b/c he is killed.
2. Candy's dog is and obvious symbol to Lennie. For one thing, it is a companion to Candy as Lennie is to George. Also, the dog's death may have been Candy's fault (read my answer in the question linked below) just like Lennie's death may be George's fault.
I think Steinbeck definitely used symbols well in this story. He also used stereotypes to give messages about certain things. People may say the book is racist, sexist, etc. but everything is there for good reason.
Crooks is a stereotypical black worker from that time. He is an outcast. However, Crooks has something no one else has really. He can read and has a library. So, Steinbeck wanted to show the truth about racism.
Females in the novel aren't portrayed well. Curley's wife is seductive, Aunt Clara was a house wife, the girl in Weed was a liar, and the other females are prostitutes. Again, Steinbeck uses the stereotype to spread the message that its wrong.