Work for hire is the idea that any work produced by an employee automatically falls under the copyright of the employer. Work for hire does not generally apply to independent contractors, but there are some exceptions:
If the work has been specially ordered.
If both parties sign a written agreement specifying a "work for hire" arrangement.
If the work is (1) a contribution to a collective project, (2) part of a media project (a motion picture, audio track, etc.), (3) a translation, (4) a supplementary work, (5) a compilation, (6) an instructional piece, (7) a test, (8) an answer for a test, or (9) an atlas.