Short answers, pulled from the story.
Emperor Palpatine created Supreme Leader Snoke as a genetic strandcast, a type of clone, to act as his proxy in power. Snoke's true purpose was to lure Kylo Ren toward the dark side and help Palpatine reclaim the galaxy through the First Order.
Supreme Leader Snoke was introduced in The Force Awakens in 2015 as Supreme Leader of the First Order and master to Kylo Ren.
Andy Serkis performed Supreme Leader Snoke as a computer-generated character using motion capture in The Force Awakens and The Last Jedi. Serkis returned to briefly voice the character again in The Rise of Skywalker in 2019.
Kylo Ren killed Supreme Leader Snoke in The Last Jedi by using the Force to cut him in half with a lightsaber. Ren then declared himself the new Supreme Leader of the First Order and blamed Rey for the killing.
Snoke's design was purposefully built so his species could never be identified, and his reluctance to meet First Order underlings in person helped hide that he was an artificial being. Only a few characters, including General Pryde, knew he answered to someone higher.
Before The Rise of Skywalker in 2019, fans theorized Supreme Leader Snoke was Darth Plagueis, Palpatine himself, Gallius Rax from Chuck Wendig's Aftermath novel trilogy, or Ezra Bridger from Star Wars Rebels. The Rise of Skywalker instead revealed him as a puppet created by Palpatine.