The answer to this question can be simple and yet can quickly become involved.  The author of an eligible work (original work of authorship fixed in a tangible medium of expression) gains a copyright in the work (even without federal registration or copyright notice) simply on the basis of authorship.  However, determining authorship of a copyright can become an involved question if multiple parties contributed to the creation.  In addition, the creator of an eligible work may not own the copyright in the work or be considered its author if the creator was employed or commissioned by another party to create the work.