A sackbut is nothing more than an early trombone! The term "sackbut" came from the French saqueboute, meaning to pull ("saquier") and thrust, or push ("buter"). This word also referred to a lance-like weapon used in the French military, armed with an iron crook, which could pull an enemy off his horse. Today, the word "sackbut" is used merely to distinguish the early trombone from its larger, louder grandchildren.
That being said, there are a lot of differences between a modern trombone and a sackbut! First, the sackbut is smaller, with thinner metal and a conical bell. Trombones gradually grew larger to accommodate the growing size of the symphony orchestra, resulting in our modern trombone with thick metal and a wide, flared bell. Sackbut bells have very little flare at all, allowing the horns to play at a softer, subtler dynamic, with more stylized articulation. This is useful for accompanying voices. You may also notice that the sackbut mouthpiece has a flat, sharp rim. This shape is important for articulation and correct tone production on the instrument. A sackbut player's goal is to sound as close to the human voice as possible!