What is the difference between Musnad and Musannaf?

A musnad collection is a collection of Hadith arranged by isnād—that is, classified according to the Companion of Muhammad to whom they were attributed. Most notable of these was the musnad of Ahmad b. Hanbal.

A muṣannaf collection is a collection of grouped ahadith, arranged in accordance to subject matter. Most notable of these is the Musannaf of Abdurrazzaaq or the Musannaf of Ibn Abī Shaybah.

However, it also includes the six canonical works which are collections of grouped ahadith, arranged by subject matter.