The Warrior caste was a group within the Klingon Empire which consisted of soldiers and formed the bulk of the Klingon military.

By the 22nd century, the Klingon warrior caste had formed the bulk of Klingon society with many young Klingons seeking glory in combat resulting in it becoming more prominent. This clouded the view of other races which included Humans who believed that all Klingons were warriors. (ENT episode: "Judgment")

This suggests that prior to that period, the Klingons consisted of other castes but as to what they were, its not known.

Fifteen generations of Martok's family had served the Empire as soldiers. However, his father Urthog wanted him to become an officer. (DS9 episode: "Once More Unto the Breach")