A Blockchain is simply a ledger of every transaction that has ever taken place using a specific currency. There are a number of different cryptocurrenies currently in use and each of them has their transaction data written to their own blockchain.
Here is a video which may help you understand blockchains better
A blockchain explorer is a tool that allows anyone to see information stored on that blockchain. For instance the CounterpartyChain.io blockchain explorer allows anyone to see information on assets and help visualize the flow of assets between users on the Counterparty network.