The peers are the physical layer where the ledger data is stored and the chaincode is processed. A blockchain network is comprised primarily of a set of peer nodes. Every peer maintains its own copy of the shared ledger and is certified by a single MSP. The peer can have two roles: endorsing nodes or committing nodes.
The endorsing node processes transaction proposals and it returns the signed result to the client.
The ordering service sends a block of transactions to the committing node. The committing node validates if the data is in a consistent state. Once verified, it commits the transaction in the ledger and updates the world state in store data.