The Wikipedia has an excellent definition for it that covers most of the points that we are going to treat:
Copyright is a legal right created by the law of a country that grants the creator of an original work exclusive rights for its use and distribution. This is usually only for a limited time. The exclusive rights are not absolute but limited by limitations and exceptions to copyright law, including fair use. A major limitation on copyright is that copyright protects only the original expression of ideas, and not the underlying ideas themselves.
The moment the author stamps the name into the creation, he/she automatically reserves all of the rights for use, modification and distribution. That easy. Even though each country has specific laws and rules for copyrights, most of the countries work under the same copyright frameworks such as the Berne Convention which recognises the author with the exclusive rights of authorisation for:
the right to translate,
the right to make adaptations and arrangements of the work,
the right to perform in public dramatic, dramatico-musical and musical works,
the right to recite literary works in public,
the right to communicate to the public the performance of such works,
the right to broadcast (with the possibility that a Contracting State may provide for a mere right to equitable remuneration instead of a right of authorization),
the right to make reproductions in any manner or form (with the possibility that a Contracting State may permit, in certain special cases, reproduction without authorization, provided that the reproduction does not conflict with the normal exploitation of the work and does not unreasonably prejudice the legitimate interests of the author; and the possibility that a Contracting State may provide, in the case of sound recordings of musical works, for a right to equitable remuneration),
the right to use the work as a basis for an audiovisual work, and the right to reproduce, distribute, perform in public or communicate to the public that audiovisual work.
When some creation is copyrighted then we understand that all rights are reserved by the author. For any use we want to make with its property we’ll need explicit permission of the copyright holders.
Now we find ourselves making a lesson and we want to reutilise content we’ve found in a book, or a website, or a video or any other place. What should we check? How should we proceed?