Style fills in the gray areas of grammar and usage. Grammar is the way in which language is structured, the rules that are the foundation of that structure and the study of those rules. Usage describes the way a language is used by native speakers, it is the application of grammar rules at a local level. Between grammar and usage there are many areas of English language use that are up for debate. Style steps into this fray to bring some order to these conflicts.
Style is a set of preferences in language and punctuation usage, it standardizes the output of a particular publisher or industry. There are many style guides. In order to know which style guide it is appropriate to use, consider your audience.
The AP Stylebook, composed by American journalists who write for the Associated Press is used by most American newspapers, though large newspapers such as The New York Times have their own style guides. The Chicago Manual of Style, published by the University of Chicago Press is used by most American publishers. However, many publishers also have in-house style books. The MLA Style Manual, published by the Modern Language Association of America, is a more recent addition to the style guide pantheon. It is used when writing academic papers. Even writers who only publish online have a style guide: The Yahoo! Style Guide, written by employees of Yahoo! This guide recognizes the more relaxed style of web writing.