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. (©)