Proof of Work (POW) is “Mining” using hardwares like Processors, GPUs, ASICs or even HDD/RAM.
Proof of Stake (POS) selection can be randomized depending on the “stake” which is directly or indirectly based on the holder’s total number of coins in possession.
Proof of stake (PoS) is a type of algorithm by which a cryptocurrency blockchain network aims to achieve distributed consensus.
POW = block validator is the one who provide most of the work (which is computation power)
POS = block validator is the one who provide most of the coins at stake.
Full explanation here