Short answers, pulled from the story.
Jason was an ancient Greek mythological hero and the leader of the Argonauts. He was the son of Aeson, the rightful king of Iolcos, and was married to the sorceress Medea, granddaughter of the sun god Helios. His quest for the Golden Fleece is the central story of his myth.
The Golden Fleece was a sacred object owned by King Aeetes of Colchis, on what is now the Black Sea coast of Georgia. Aeetes had received it from a man named Phrixus. Jason was sent to retrieve it by Pelias, who promised Jason the throne of Iolcus in exchange for completing the quest.
The Argonauts were a group of Greek heroes who sailed with Jason aboard the ship Argo, built by a crew member named Argus. The crew included Heracles, Orpheus, Atalanta, Peleus, the Dioscuri Castor and Polydeuces, the seer Idmon, the helmsman Tiphys, and the winged brothers Calais and Zetes, among others.
Medea, a sorceress and daughter of King Aeetes, gave Jason an ointment to protect him from fire-breathing oxen called the Khalkotauroi, warned him how to defeat warriors that sprouted from dragon's teeth, and provided a herb-based potion that put the sleepless dragon guarding the fleece to sleep. Hera had arranged for Medea to fall in love with Jason through the intervention of Eros.
Jason died lonely and unhappy after losing the favor of Hera for breaking his vow to love Medea forever. He was resting under the stern of the rotting Argo when it fell on him and killed him instantly.
Dante placed Jason in the Eighth Circle of Hell, in the first ditch known as Bolgia 1. Jason is seen by Dante and his guide Virgil being driven to march through the circle for eternity while devils whip him. Dante classified him among the panderers and seducers, possibly for his seduction and abandonment of Medea.