Chicago is the style manual of the University of Chicago Press. This manual is used by many publishers, editors, and professors throughout the English-speaking world as a guide to manuscript preparation. Note: The Chicago Manual of Style allows for two styles: Notes-Bibliography (usually used in natural and social sciences) and Author-Date (usually used in the humanities). Ask your instructor which style you are expected to use.
Turabian is an abridged version of the Chicago Manual of Style written for college and high school students. It includes more information about formatting papers and the research process than the Chicago manual. If you can't find a citation format for a source in Turabian, look in the Chicago manual.
Chicago Manual of Style Formatting and Style Guide available from the Online Writing Lab (OWL) at Purdue, provides extensive examples for citing common and more obscure sources in the Chicago Notes-Bibliography style and guidelines for formatting a paper.
Chicago Manual of Style Q&A, from the University of Chicago Press, discusses particular situations and examples that are not covered in detail in the manual.
Turabian Citation Guide, from the University of Chicago Press, provides samples of in-text citations and bibliography/reference list entries in Notes-Bibliography and Author-Date styles.