Once the work is put into a tangible form--a form in which others can read it, see it, or use it in some way--it is given an instant natural copyright, granted to the person who created the work. That person then has the option of getting a registered copyright, which allows more protection in court should copyright infringement occur. If the creator doesn't choose to register the copyright, a natural copyright is still in place on the work.

Only the creator of the work at this point has the right to grant anyone else permission to use the work, in any fixed means, other than the exclusions granted by Fair Use. The copyright holder can choose to transfer the copyright to someone else (usually, but not always for monetary exchange), or to license others to use the work. If the copyright is transferred to someone else, then only the copyright owner has the right to grant permission.

Any use of the work--not included in the Fair Use exclusion--that is used without the express permission of the copyright owner is considered copyright infringement.

Plagiarism is using someone else's works as though it were your own, by either passing it off as though you wrote it or by failing to properly attribute the work, letting others believe you completed the work yourself. Plagiarism also doesn't require an exact copy of the work to be considered plagiarized, as long as a good portion of the work is obviously from someone else.

When copyright infringement and plagiarism go together, it's usually because someone copied someone else's creative works, in whole or in part, and then allowed it to appear they were the original creator.