An author's message in a work of fiction is more often than not referred to as the theme. S.E. Hinton has a few themes present in The Outsiders, but I feel the most powerful theme/message is that there is no point in class conflicts.
The beginning of the novel clearly lays out that there is a class division in Ponyboy's world. There are Greasers, and there are Socs. The two gangs hate each other, but the only obvious difference between the two groups is financial well being. The feud is hard on both groups. Johnny gets beat to within an inch of his life. Bob actually does lose his life. Ponyboy is likely forever scarred by the events of the book.
Hinton begins to lay out her main theme early in the book, when she has Cherry teach Ponyboy that "things are rough all over." Pony believes her, and it is the start of him seeing the Socs as people similar to him.
It seemed funny to me that the sunset she saw from her patio and the one I saw from the back steps was the same one. Maybe the two different worlds we lived in weren't so different. We saw the same sunset.
By the end of the novel, Ponyboy has learned that each group is more similar to each other than different. He has also learned that their feud is more destructive than anything else; therefore, it is pointless to continue fighting.