Citing a source means that you show, within the body of your text, that you took words, ideas, figures, images, etc. from another place.
Citations are a short way to uniquely identify a published work (e.g. book, article, chapter, web site). They are found in bibliographies and reference lists and are also collected in article and book databases.
Citations consist of standard elements, and contain all the information necessary to identify and track down publications, including:
titles and/or containers
date of publication
volume and issue numbers (for articles)
DOI (now encouraged by MLA 8th edition)
Below is an example of an article citation using MLA style, 8th Edition using the following element details:
Author - William D. Nordhaus
Article Title - Global Warming Economics
Source Title / Container 1 - Science
Volume and Issue - volume 294, number 5545
Publication Date - 2001
Page numbers - 1283 - 84
DOI - 10.1126/science.1065007
Database / Subscription Service / Container 2 - EBSCOhost
MLA in-text (or parenthetical) citations follows an Author-Page Number style. Purdue Owl Writing Lab has a good tutorial on MLA in-text citation basics.