Plagiarism is publishing articles from others as your own without permission as Arthur pointed out in his comment. I have seen it many times on several networks. People post articles of others without mentioning the source. I do not think it is OK to do so. It can backfire on you if you publish content that is not your own.

Many article directories explicitly say that you may use the articles but that you must mention the source and give the original author credit. If you read their TOS you can be in big trouble if you don’t; the original author might start a law suit against you.

What about PLR articles where the author is unknown?

Well you won’t get in any legal trouble. Still I recommended adding a note that it is a PLR article or just write as source “author unknown”. Why? What if you get feedback from people asking you questions and you can’t answer them? You would look pretty dumb. And why should you use a PLR article if you know much about the subject yourself? It is much better than to write your own articles on that subject. They will rank much better in search engines than copy/paste articles or articles from an article spinner in my opinion; SEO experts correct me if I am wrong.

Don’t think people won’t find out. People that read a lot will remember if they saw an article before elsewhere. There are also services that can search for duplicate content.

Sometimes it is so obvious that an article is plagiarism that it makes me laugh (although it is really not funny at all). When somebody who’s native language is not English and who’s comments on articles are very poor English, or has a profile in very poor English, is very unlikely to write a number of articles in perfect English unless he hired a ghost writer. 9 of 10 times the same content can be found when you perform a search.

So be honest and don’t publish content of others under your own name unless you have explicit permission of the owner of the original owner like is the case of some syndicated posts on this blog. Don’t be a copy cat and mention your sources, people will respect you for that.

If you are having trouble writing articles i would like to refer to the article “Are you having trouble writing quality articles that let your readers crave for more?”