Pon farr was the Vulcan time of mating, and was thus, a major part of the reproductive cycle. The pon farr occured approximately once every seven years during adulthood for both sexes. During pon farr, a Vulcan loses all emotional control and must either mate or die. (TOS episode & novelization: Amok Time; TOS movie, novelization & comic adaptation: The Search for Spock)
During the onset of the pon farr, the Vulcan underwent a neurochemical imbalance which took the form of madness, which later developed into plak tow. Unless the pon farr was satisfied, the Vulcan would die within eight days of the process beginning. (TOS episode: "Amok Time"; VOY episode: "Blood Fever")
In the days before Vulcans embraced Surak's teachings of peace and logic, Vulcans undergoing the pon farr would often kill in order to win a mate. After the Time of Awakening however, the koon-ut-kal-if-fee was adopted, and Vulcans became telepathically bonded when they were young (TOS episode: "Amok Time") during the betrothal ceremony, or Koon-ut-la. (TOS novel: Mind Meld) The telepathic link led the couple to one another when one of them entered pon farr. The bonding was completed when the couple consummated the marriage.
Due to the very private nature of the pon farr within Vulcan society, outsiders were not told of it, and Starfleet doctors only began to find out in the 23rd century. Even by the 2370s, medical knowledge about the pon farr was scarce. (VOY episode: "Blood Fever")
Despite being a Vulcan offshoot, the Romulans did not go through pon farr, having removed it from their genome with genetic engineering after the Sundering. (TOS - Rihannsu novel: The Romulan Way) However, this was not true in all instances. A Romulan boy who was bonded with Spock’s young Vulcan female relative did go through pon farr, demonstrating that Romulans and Vulcans are descended from the same stock. (TOS novel: Mind Meld)