A protocol is a standard composed of a set of rules and conventions used for a given purpose. Let us take a look at what some of the conventions and rules for a blockchain look like.
One subset of the rules regards maintaining the ledger. Each node keeps a copy of the blockchain and verifies every transaction it receives. Once a transaction becomes verified it is then saved in the mempool (short for memory pool) with all the other transactions that are not included in a block. When a node receives a new block from its peers, it checks the validity of the block first. If it is valid, then it is added to the local copy of the blockchain and all transactions included in the block are removed from the mempool. The mempool will only ever contain unconfirmed transactions.
Another subset of rules is concerned with the structure of a valid block. A block has a block header that contains information about the version of the bitcoin client it was created with, a reference to it’s preceding block, a sort of summary of all transactions that are contained in the block (the Merkle Root), a timestamp and some other useful information.
Following the block header, the block contains all the transactions that were included.
Now there needs to be a set of rules describing what an individual transaction must look like. We will talk about transactions in detail in a dedicated article but for now, it should suffice to know that a transaction needs to include the sender, the recipient, the amount of the transfer, and a digital signature. The digital signature is a way to authorize the spending of your funds. The private key of a user creates this digital signature. We explain this process further in our article on public-key cryptography.