Short answers, pulled from the story.
Pancharatra is a Sanskrit compound meaning "five nights" (pañca: five, rātra: nights). The term has been interpreted variously as "five knowledges," "five systems," "five studies," and "five rituals." Scholar Jan Gonda suggested that "nights" may be a metaphor for inner darkness, though he acknowledged the exact semantic shift remains unclear.
Pancharatra has likely roots in the late 3rd century BCE, when it formed as a religious movement around the ideas of a sage named Narayana. The earliest known use of the word Pancharatra appears in section 7.1.10 of the Taittiriya Samhita, a Vedic text.
The 11th-century Sri Vaishnavism scholar Ramanuja, born into the Pancharatra tradition, developed a qualified monism doctrine called Vishishtadvaita that integrated Pancharatra ideas with the monistic strands of the Vedas. He argued that Vishnu as understood in Pancharatra is identical to Vedanta's Brahman, directly countering the 8th-century Adi Shankara's claim that the tradition was non-Vedic.
Ramanuja taught that the absolute deity manifests in five aspects: Para (the invisible, eternal supreme), Vyuha (the invisible, impermanent supreme in form), Vibhava or Avatara (incarnations across the yugas, including the Dashavatara), Antaryamin (an aspect inferred but not directly perceived), and Archa (the visible icon form consecrated in temples or kept at home).
The Pancharatra Agamas comprise more than 200 texts, most likely composed between 600 CE and 850 CE. The tradition itself claims 108 samhitas, but the texts list over 200. The Kapinjala Samhita alone mentions as many as 215 Pancharatra Samhitas, and many have been lost entirely.
The Pancharatra is considered authoritative by four sampradayas: the Sri Vaishnava sampradaya (Vishishtadvaita), the Madhva sampradaya (Dvaita), the Vallabha sampradaya (Shuddhadvita), and the Gaudiya sampradaya. Barbara Holdrege, a comparative historian of religions, has documented how the Pancharatra doctrines influenced both Sri Vaishnavism and Gaudiya Vaishnavism, albeit in different ways.