“An intellectual reference to a published or unpublished source by quoting of a book, author or an existing publication in support of a fact.”

More precisely, a citation is an abbreviated alphanumeric expression (e.g. [Newell84]) embedded in the body of an intellectual work that denotes an entry in the bibliographic references section of the work for the purpose of acknowledging the relevance of the works of others to the topic of discussion at the spot where the citation appears. Generally the combination of both the in-body citation and the bibliographic entry constitutes what is commonly thought of as a citation (whereas bibliographic entries by themselves are not).
Forms and Types of Citations

The forms of citations generally subscribe to one of the generally accepted citations systems:

MLA: Modern Language Association of America
ASA: American Sociological Association
APA: American Psychological Association
AAA: American Anthropological Association
CSE: Council of Science Editors
CBE: Council of Biology Editors

Each of these citation systems has its respective advantages and disadvantages relative to the trade-offs of being informative (but not too disruptive) and thus should be chosen relative to the needs of the type of publication being crafted. Editors and academics usually specify the preferred citation system to use.

Bibliographies, and other list-like compilations of references, are generally not considered citations because they do not fulfill the true spirit of the term: deliberate acknowledgment by other authors of the priority of one’s ideas.
Ways of Using Citation

You can incorporate someone else’s work into your own in three ways:

Quotations must be identical as in the source consulted. Only quote phrases, lines, or passages relevant to your subject matter and do not change spellings or punctuation of the original quotes.

Paraphrasing involves writing your passage, phrase by phrase from the source into your own words. Your passage should be of equal length or shorter than the original passage. Paraphrasing means a complete rewrite of the consulted source passage and not just rearrangement of words.

Summarizing includes putting the main idea(s) of a passage into your own words. Summaries are much shorter than the original source passage. Make sure to not change or alter the original meaning of the passage while summarizing main idea(s).

All three methods must identify and credit the sources used in the paper and allow others to access and retrieve this material.

Unique Identifiers

Along with typical information on author(s), date of publication, title and page numbers, citations also include unique identifiers often used for specific kinds of reference works:

International Standard Book Number (ISBN): Used for citations of books
Serial Item and Contribution Identifier (SICI): Used for specific volumes, journal articles or other parts of a periodical
Digital Object Identifier (DOI): Used for electronic documents and sources
PubMed Identifier (PMID): Used for biomedical research articles

Author's Bio: 

Amy Dyslex has written this article. Visit www.writeawriting.com to read more about citation guidelines.