Motor yachts and powerboats are typically defined as not sailing vessels but vessels powered by an internal combustion engine. A speedboat is a smaller powerboat used in races or for weekend outings. They are used for such things as ski boats to pull water skiers.
There are three common varieties of engines, inboard, inboard/outdrive, and outboard.
There are various classificatons of yachts. Luxury motor yachts and super-yachts are considered by many to be 75' and above in length. These very expensive privately owned yacht are almost always professionally crewed.