Plagiarism is "the unauthorized use or close imitation of the language and thoughts of another author and the representation of them as one's own original work."
Dictionary.com Unabridged (v 1.1). Retrieved February 03, 2009, from Dictionary.com website: http://dictionary.reference.com/browse/plagiarism
If you buy a paper from a web site, written by someone else and try to pass it off as your writing, that's obviously intentional plagiarism. Not all plagiarism may be intentional, however.
If you copy and paste all or part of a sentence from an online source and don't attribute it to the person who wrote it, that's plagiarism.
If you use the same basic sentences from someone elses writing and simply substitute a word here and there to make it sound different, that's plagiarism.