A duplicate content is when a website or blog have a similar content as with any other websites in the worlds. Contents which are almost likely similar are also considered as duplicates.
Google is penalizing these content grabbers and puts down their ranks and even omits that website altogether. But it seems Google bots is not wise enough to pick which contents are maliciously copied or not and sometimes sees the originals as copies.
An article on the link below, discusses more about content duplication and how you could prevent it.
Link: news How to Deal with Duplicate Content
