Plain English definition
Work for hire makes the client the legal author — not just the owner — of the work. The copyright vests in the client from creation, with a 95-year term rather than the creator's life plus 70 years. It also means the creator has no termination rights to reclaim the work after 35 years.
When it applies to contractors
Work for hire applies to independent contractors ONLY for work that falls into one of nine statutory categories (contributions to collective works, translations, instructional texts, tests, supplementary works, compilations, audiovisual works, atlases, and motion pictures) AND only with a signed written agreement.
What happens without the written agreement
If the work qualifies under one of the nine categories but there's no signed written agreement, work for hire doesn't apply. The creator retains copyright. The client should use a copyright assignment clause instead — which is valid for any type of work.