Copyright refers to the legal right of the owner of intellectual property. In simpler terms, copyright is the right to copy. This means that the original creators of products and anyone they give authorization to are the only ones with the exclusive right to reproduce the work.
Copyright law gives creators of original material the exclusive right to further use and duplicate that material for a given amount of time. Once a copyright expires, the copyrighted item becomes public domain.
Under U.S. law, the initial copyright holder is the author of the work. In most cases, copyright law treats the creator(s) of the work as the author(s). Copyright is automatic; it applies to the work as soon as it is fixed(or recorded) in some way. If one writes an essay or article, they are the owner of that article unless and until they contract away their rights (such as in a publishing agreement).
If multiple people create a work, only those who have contributed copyrightable elements are considered authors for the purpose of copyright law. Coming up with the idea for the work alone is not enough to be an author. See "joint ownership of a copyrighted work" for more information about multiple authors.
If someone creates a work as an employee (or in certain cases, as a contractor), that person’s employer is considered the author of the work. See "works made for hire" for more information.