Copyright is a legally established right that protects the rights of creators of artistic creations such as texts, music scores, paintings, designs and sculptures.
International copyright concepts are covered by the Berne Convention. All signatories to the Berne Convention recognise basically the same rules on copyright, though there are some differences in individual laws since they are drawn up by each national government.
The purpose of copyright is to prevent artistic works being freely copied or adapted, and thereby to encourage the production of new works by enabling creators to benefit from their efforts.
Copyright arises automatically when an new work is created. It does not have to be registered.
The copyright in a work belongs to the individual creator, and he can transfer or sell it in whole or in part with conditions attached.
When a company employee creates a work, the copyright usually belongs to the company under employment contract terms.
The general term of copyright is Life + 70 years, or 90 years from date of creation for a corporation. Once the term has expired, the work enters the public domain and can be freely copied and adapted.
The core point of copyright is that a copyrighted work should not be copied without permission of the creator. There are certain exceptions to this, under the heading of Fair Dealing or Fair Use.
Fair Dealing includes comment, criticism, and educational uses. It should be recognised that the Fair Dealing exceptions are not absolute rights to copy works.
The full explanation of copyright and fair dealing as it applies within legal jurisdictions can be found at government copyright agency web sites.
Here is a nice basic copyright FAQ.
Finally, the Wikipedia article offers a good layman’s overview of copyright.
I am not a lawyer.