When doing research it is often necessary to consult other research. When you use an idea or statistic that comes from another source, you need to reference that source; failure to do so is considered plagiarism. The citation is the reference to that original source (i.e., book, paper, article, web site, etc.). Citing sources gives credit to the original author, enables readers to find the original source and strengthens your work.
A citation usually includes five important pieces of information:
Where the work comes from. For books, this would be the publisher; for journal articles, this would be the title of the journal, for government documents, the agency name, for technical reports, the corporation or agency, etc.
Any information that's needed to find the work within/from the source; this varies with format. For books the location would the city of the publisher; for journal articles it would be the volume, issue and pages; for items only published online this could be an URL or a DOI (digital object identifier).
How these elements are ordered in the citation and what punctuation, spacing, and font style (italics, bold) are used depends on the citation style you are asked to use.
List of Style Guides: