Miners & Validators
Miners produce blocks on Proof of Work chains; validators do the equivalent job on Proof of Stake chains.
In depth
Both check that transactions follow the rules, assemble them into blocks and propagate the result. The difference is what they put at risk: miners spend electricity and hardware, validators lock up coins that can be confiscated. Both are rewarded with new coins and fees for honest work.
Why it matters
They are the people (and machines) actually writing the ledger you rely on.