This is not legal advice.
Copyright is automatic, you don't have to express it (but it helps) or do anything special to assert it. It applies to the expression of ideas, not to the ideas themselves. In general, it lasts for between 50 and 70 years, depending on the context and jurisdiction. If you are employed, the copyright probably belongs to the employer. If you publish, you probably have to transfer copyright to the publisher (they will make you sign something). Copyright confers the exclusive (important word!) right to reproduce or perform the work. If you want to use someone else's copyrighted work, you must get their permission, preferably in writing.
- Public domain
- means that the work has fallen out of copyright, or has been relinquished by the creator. The work is effectively owned by the public. The Mona Lisa, old photos, etc, are in the public domain. Many government creations are released into the public domain. You can do whatever you like with public domain work, including change it.
- is a way to allow others to reproduce work. Licenses might cost money. Today, the most common type of licensing grants some kind of permission to the user. The two most common models are permissive and non-permissive licenses.
- Permissive licenses
- sometimes called copyfree, allow the user to reproduce or modify the work and then release it under different terms, and even sell it. An example is the Creative Commons Attribution license, or CC-BY, which requires only that the creator is acknowledged in any future versions of the material.
- Non-permissive licenses
- sometimes called copyleft, are more restrictive. Reproduced or modified work must be shared under the same terms as the original; for this reason, these licenses are sometimes called viral. Other restrictions might include non-commercial use only, or no derivatives allowed. Some people regard these licenses as truly free and open. Others think they are less free than permissive licenses, because of the restrictions. An example is the Creative Commons Attribution Share Alike license, or CC-BY-SA; Wikipedia uses this.