If you have been an active forum reader for the past few weeks, you would have noticed a lot of posts (and resulting changes) about copyright infringement.  Putting aside all of the legal ramifications, there is another good reason why you should never copy and paste material found elsewhere on the web. 

The major search engines are always looking to streamline their index.  With trillions and trillons of pages indexed, the cost to run and upkeep the big three is staggering.  As consumers, we never give it a thought.  One of the ways that these companies can reduce that number while providing accurate relevant results is to not include information that is already listed.  Sites that have been delisted for this are said to have incurred a duplicate content penalty (in se lingo).  You ask but what about article sites?  Good question.  Search engines are selective about which sites they will remove for duplicate content.  When the content is on the same domain, its an easy call for them (i.e. the same information on multiple pages).  Normally they'll keep one page and ditch the rest. Same thing for different domains.  Say site A that has been on the web for years is copied by site B.  Site B posts the same text as site A, hoping to get the same great rankings that site A enjoys.  Site B is then indexed by the major 3 engines.  Sometimes some time will pass before the duplicate content is caught and removed.   If enough duplicate content is found the entire site is sometimes removed as well. 

Sometimes its necessary to post something verbatim, especially when citing an article or other source.  You can avoid running the risk of a penalty by making sure the page has addition unique information on it.  The goal would be when looking at the page as a whole, the posted material is less than half the content (the less the better).

-Charles