Copyright is a system which protects a persons official
work.
When someone copies, reproduces, or does anything to a protected
work that recreates any original work in it, they are going against the rules of copyright. The original creator of that piece of
work can sue the person who broke the rule against his
work.
Remember that copyright does not protect even official
work forever. When it is no longer protected by copyright, it becomes part of the public domain. This means people can now copy this piece of
work.
You can get permission from the owner to copy the piece of work, and if he/she says
yes, you can copy their work without any trouble.
Another thing you should know about copyright is "fair use." Fair use is a rule in which works can be used for purposes, like education, parodies, and more.
Other Fact:
There is even a symbol for copyright. (©)