Blockchain technology underpins cryptocurrencies like Bitcoin, while Web3 is the protocol that allows decentralized applications to run on top of a blockchain.
Blockchains are like the internet, while Web3 is the deployment and functioning agent. With blockchains, one can create a decentralized network where users can interact directly with each other without having to go through a third party.
The blockchain is, at its essence, a decentralized data structure, that demands a sequential series of required data, in order to fulfill any next step. This means that the data shared is agnostic in policy, so long as the data requirements imbedded in any agreement is met.
The blockchain is a decentralized entity. There is no state or international governance or legal enforcement. There is only what is required through any given agreement and the related code. Every "block" is an equation of data that has been instilled into the chain of events. In order to have whatever data move forward to the next block, it is required to fulfill certain data requirements. If they are met in that block, they will then move along the chain, until all data and requirements have been met.