Taking chunks from a source and letting no one know about it as you pass it off as your own.
Plagiarism is the soulless act of copying chunks of someone else's work, slapping your name on it and passing yourself of as the original author.
That's just not cricket and there is no excuse for such poor form.
If the answer you're posting is answered on an external site, either quote a small portion or rephrase it in your own words. In either case, provide a link back for attribution. Because you don't want to be a jerk.
Not the same as a copyright violation, but belongs in the same ethics bucket.
Acts of plagiarism can be handled by moderators. For copyright infringement issues, please contact Stack Exchange.