Short answers, pulled from the story.
A mining pool is a group of cryptocurrency miners who combine their processing power over a network to improve their collective odds of finding a block and earning a reward. Miners share their computational resources and split the reward proportionally based on the work each contributed, measured through a system of shares.
The first mining pool, called Slush, launched in November 2010. It introduced what became known as Slush's system, or pooled mining, where older shares from early in a round are weighted less than more recent ones.
A share is a partial proof-of-work submitted by a miner that meets a lower difficulty threshold than a full block solution. Pools use shares to measure each miner's contribution; one of those shares will eventually also satisfy the full block difficulty and become the actual block solution.
Pay-per-Share (PPS) pays miners an instant, guaranteed amount for each share submitted, transferring most financial risk to the pool operator. Pay-per-last-N-shares (PPLNS) calculates each miner's payout based only on the last N shares submitted, meaning reward size depends on how short or long the round was.
P2Pool is a peer-to-peer mining pool that removes the central server entirely by having miners work on a separate side blockchain called a share chain, producing one share block every 30 seconds. Because there is no single pool operator, there is no central point of failure or opportunity for the operator to manipulate rewards.
From 2022 through 2024, Foundry USA Pool and AntPool consistently held the largest shares of Bitcoin's global hashrate. Foundry USA Pool, a subsidiary of Digital Currency Group, first became one of the largest pools by hashrate in 2021.