Plagiarism means using other peoples words, facts or ideas without giving credit properly. Contrary to what some people imagine, it is possible to plagiarize without meaning to. The consequences can be severe, so its a good idea to learn the proper way to use others words in order to avoid the problem in the first place.
A simple but profound rule will help eliminate most problems with unintentional plagiarism:
quote exactly (word for word, comma for comma), in quotation marks ( )
Rewrite completely in your own words.
For more on avoiding plagiarism, visit these excellent sites: