In the Blockchain, all transactions are publicly announced to all nodes, as there is no central authority who validate transactions. The validation system is called Proof of Work.
This paragraph is somewhat misleading. The POW isn't used for validation, it's there to prevent double spending and modification of the chain. Validation of transactions in the block is done via cryptographic signatures and has nothing to do with POW. I would emphasize this more, as it's abit unclear from your post what is the purpose of POW in Bitcoin.