In the Extraordinary Form, there are two primary types of Masses celebrated: the Low Mass and the Solemn Mass (sometimes referred to as High Mass).
Low Mass is celebrated by a priest assisted by one or more altar boys. The prayers are spoken in the low voice, which is almost inaudible, in imitation of the ancient Jewish liturgies given by God to Moses, and emphasizing the fact that we are faced with a profound mystery.
Solemn Mass is the full ceremonial form of the traditional Roman Mass, celebrated by a priest with a deacon and a subdeacon, requiring most of the parts of the Mass to be sung, and the use of incense. The parts assigned to the deacon and subdeacon are often done by priests in vestments proper to those roles. (source)