Copyright is a term encompassing a wide range of laws designed to protect inventors and creators of original work from having such work resold without permission, falsely attributed, or used in some other way that is harmful to the creator or their ability to market the product. It's scope often intersects those of intellectual property and patent law. Plagiarists risk trouble for violating copyright laws.
The first appearance of a copyright law dates back to the UK in 1709, when the Statute of Anne was created. It protected the authors of books, maps and drafts from their works being copied for up to 28 years (14 years + an optional extension for another 14 years). In 1790, a similar law was also accepted by the United States Congress. After that, length of the copyright term, as well as its scope, were extended multiple times; as a result, nowadays a work is protected by copyright from being published until 70 years after the author's death (or 95 years in case of works made by companies).
While the increasing availability and scope of mass media has called the entire Philosophy of copyright into debate, there is general agreement that at its best, copyright protects everyone from the small-time blogger to the heavily-funded research collaboration from having their creative works blatantly abused, and at its worst, it has been used to prevent legitimate art and research from being pursued, even if such work is not intended to be marketed.
Despite many on-line still debating to the degree of how exactly copyright should be reformed (or not), many agree that today's long term and wide scope of copyright rewards publishers significantly more than both people or the authors of their works.
Copyright laws sometimes unpopular because big money making corporations will sue you for copying a movie they made to share with friends. But at least we have it, because if we have Eric Bauman, we need copyright.