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
It's obviously plagiarism when you buy a paper from a web site, written by someone else, and try to pass it off as your own. Not all plagiarism may be intentional. For example,
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 you are plagiarizing.
If you use the same basic sentences from someone else's writing and simply substitute a word here and there to make it sound different you are plagiarizing.
It doesn't matter whether you intended to plagiarize! You are still plagiarizing and can be held accountable for that plagiarism.