Copyright is an intellectual property right that grants the creator of an original work a set of exclusive rights (generally, to use and distribute the work) for a limited period of time. The purpose of copyright is to encourage and support creativity in the arts and sciences by giving the creator (i.e. the author of a book, the painter of a painting, etc) a means of receiving compensation for the intellectual effort.
Traditionally, copyright was said to be territorial. This meant that copyright did not extend beyond a particular country's jurisdiction unless that country was part of an international agreement. Today, this is much less relevant because the vast majority of countries are party to at least one such agreement.
While all countries have a unique set of copyright laws, most countries recognize copyright in any completed work (without a requirement of formal copyright registration), copyright is enforced as a civil matter, and the duration of copyright is the entire life of the creator plus 50 - 100 years.
Registering a Copyright
The purpose of copyright registration is to create a verifiable record of the date and content of the work in question, which is extremely helpful in cases of copyright infringement or plagiarism. International convention, primarily the Berne Convention, provides international rights without the requirement for national registration. However, the United States is still an outlier in that it provides legal advantages for registering works of U.S. origin.