A citation is a referral to an information source. It is usually provided as a combination of title, author, date and location (e.g. URL). Citing your sources is the best way to avoid plagiarism. Plagiarism can be deliberate - knowingly using someone else's work as your own. It can also be inadvertent, sometimes plagiarism accusations are simply the result of not following a specific style properly. The specific style you are using (e.g. APA) will dictate the details, see the citation page on this guide for help with a particular style.
Whenever you quote, paraphrase or summarize an article, book, website or any other information source, you must provide a citation - that is, give the reader enough information to find the original work. Title, author or creator name, creation date and location information (e.g. URL) are usually provided in a citation.
But wait - there is an exception. If something is considered to be common knowledge, you don't need to cite a source. Common knowledge is comprised of widely known and accepted facts. E.g. Ocean water is salt water; grass is green, the earth is round, etc. If you're not sure, just use a citation - always err on the side of caution!
Wikipedia's credibility went up when they changed their editorial policy to include citations. That's all about authority. Citing is providing evidence of how you know what you know. It lets other readers see the sources of your information.