“The Merge” refers of an hard fork happened in September 2022 where the Proof of Work was changed in Proof of Stake. This because the difficulty of Ethereum’s PoW increased exponentially every 100,000 blocks, which means that at a certain point the difficulty will become impossibility.
Beacon
Before the hard fork, Ethereum was compatible with both PoW and PoS.
The beacon chain is a separate blockchain that runs in parallel with the existing Ethereum 1.0 chain. Its primary purpose is to manage the PoS protocol, coordinating validators and proposing blocks for the shard chains.
Shard chains
Shard chains is another update in Ethereum 2.0, where secondary blockchain branches are maintained by smaller networks, and synchronized with the primary chain from time to time. Shard chains are way faster because of the smaller size of the subnetworks.
Stake pools
Just like mining pools, if a single account doesn’t have the funds to become a validator, multiple accounts can join a pool where they divide the stake and the rewards.