Mozart only wrote half of his most famous work.
When Mozart died in Vienna on 5 December 1791, aged 35, his Requiem in D minor was unfinished: only the Introit was complete, other movements existed as sketches. His widow Constanze had it completed in secret, mainly by his pupil Franz Xaver Süssmayr, to keep the anonymous commissioner's fee. What you hear today is usually that version.
Useful: With 'masterpieces', it's fair to ask who else actually worked on them.
Source: en.wikipedia.org