Short answers, pulled from the story.
AlphaGo is a computer program that plays the board game Go, developed by DeepMind Technologies, a London-based company that is an acquired subsidiary of Google. It uses a combination of Monte Carlo tree search and deep neural networks trained through reinforcement learning.
AlphaGo beat Lee Sedol in a five-game match played on the 9th, 10th, 12th, 13th, and the 15th of March 2016 at the Four Seasons Hotel in Seoul, South Korea. The final score was 4 games to 1 in favour of AlphaGo; Lee Sedol won only the fourth game, making him the only human to beat AlphaGo in any of its 74 official games.
Move 78, played by Lee Sedol in game four, was dubbed the "divine move" by many professionals. AlphaGo's policy network had assigned that specific move such a low probability that the program could not make the correct response after it was played, causing it to lose the game. DeepMind's Aja Huang revealed in June 2016 that the team subsequently patched this logical weakness.
AlphaGo Zero was trained without any human game data. By playing only against itself, it surpassed the strength of the version that defeated Lee Sedol in three days, reached the level of AlphaGo Master in 21 days, and exceeded all previous versions within 40 days. It achieved a 100-0 record against the early competitive version of AlphaGo.
Lee Sedol announced his retirement from professional play on the 19th of November 2019, stating that he could never become the top overall player in Go due to the increasing dominance of AI. He described programs like AlphaGo as "an entity that cannot be defeated."
AlphaGo's victory was described in China as a "Sputnik moment" that convinced the Chinese government to dramatically increase funding for artificial intelligence. A 2018 paper in Nature cited AlphaGo's approach as the basis for a new method of computing potential pharmaceutical drug molecules. The DeepMind AlphaGo team also received the inaugural IJCAI Marvin Minsky Medal for Outstanding Achievements in AI in 2017.