The Pontifical High Mass really is considered historically the basis for the whole liturgy and its evolution, especially since there were no masses that were sung by exclusively priests in the early church.
It is very different then a mass celebrated only by a Priest. Almost the entire first half of the mass is said from the throne, which sits on the left side of the altar. Instead of saying Dominus Vobiscum (the Lord be with you), the Bishop says Pax Vobiscum (peace be with you). The bishop wears additional garmets above and beyond what the priest wears:
Before Vatican II, the Popes pontifical mass when he celebrated at St. Peters was even more different. It included the gospel and epistle being sung in Greek by Greek priests, because St. Peter's is actually supposed to be the Greek church in Rome (the Pope's real Cathedral being St. John Lateran). During the liturgy these Greek priests observe the rules of their right, such as a profound bow instead of kneeling. The Pope drinks the wine, which they believe is the body and blood of Jesus Christ, through a straw. Also pieces of the bread before it is turned into the body and blood of Jesus Christ is tested by a deacon before the Pope to check to see if it is poisoned.