There are many citation styles, but citations usually include information about the authors, title, and publisher of a work, and the date the work was published and/or accessed.
A Modern Language Association hosted website with information on MLA citation and related resources. The MLA Style Center does not contain the full text of the handbook, although it walks users through the process of creating an entry in the works cited list.
From the Online Writing Lab at Purdue University, this is one of the most comprehensive online citation guides available. Includes in-text citations, sample papers, works cited pages, annotated bibliographies, and dozens of examples for every possible source type.