Why is the title of the book "The Outsiders"? I want to know how the title links to the story?

The title of the book refers to the social position of the Greasers, Ponyboy's "gang".  Handicapped by their lower socio-economic status, the Greasers are looked down upon; shut out from the advantages that are open to their more wealthy counterparts, the Socs.  Although members of both groups get in trouble - the Socs "jump Greasers and wreck houses and throw beer blasts for kicks", while the Greasers "steal things...and rob gas stations and have a gang fight once in awhile" - the Greasers are more frequently identified by their transgressions against the law because they come from "the wrong side of the tracks".  They are the "outsiders", the ones for whom access to opportunity is perpetually more complicated, because of preconceptions and stereotypes (Chapter 1).

One of the central themes of the story, however, is that, underneath it all, the Greasers and the Socs are more alike than they are different.  As Cherry Valance observes to Ponyboy, "things are tough all over", and, after talking with her, Ponyboy realizes that there are individual differences within both groups, and, despite the gulf that separates the classes, they all "see the same sunset" (Chapter 2).