Chicago = Chicago Manual of Style
This citation style is also commonly used around the world. Disciplines that typically use it are arts, humanities, and history, but many other disciplines use Chicago. Supposedly, it is the oldest citation style in use, with a history spanning more than 100 years.
The current edition is the 17th edition, so make sure you format your citations in the appropriate edition format that your instructor or professor requests.
The Chicago Manual of Style [17th edition] by 
The Chicago Manual of Style [16th edition] by 
Cite Right : A Quick Guide to Citation Styles - MLA, APA, Chicago, the Sciences, Professions, and More by 
The Chicago Guide to Grammar, Usage, and Punctuation by 
The Chicago Guide to Writing about Numbers by 