In Proof-of-Stake (PoS) and Delegated Proof-of-Stake (DPoS) networks, transactions are processed by validating node who staked a significant amount of coins. This consensus mechanism has blocks added by by block producers as compared to miners under the Proof-of-Work consensus.
Validators a chosen on a rotation basis depending upon what the protocol outlines. They are the ones who maintain the ledger for the blockchain.