The blockchain is a public register in which transactions between two users belonging to the same network are stored in a secure, verifiable and permanent way. The data relating to the exchanges are saved inside cryptographic blocks, connected in a hierarchical manner to each other. This creates an endless chain of data blocks — hence the name blockchain — that allows you to trace and verify all the transactions you have ever made.
The main function of a blockchain is to certify transactions between people. In the case of Bitcoin, the blockchain serves to verify the exchange of cryptocurrency between two users, but it is only one of the many possible uses of this technological structure. In other sectors, the blockchain can certify the exchange of shares and stocks, operate as if it were a notary and “validate” a contract or make the votes cast in online voting secure and impossible to alter.
A block is the ‘current’ part of a blockchain, which records some or all of the recent transactions. Once completed, a block goes into the blockchain as a permanent database. Each time a block gets completed, a new one is generated. There is a countless number of such blocks in the blockchain, connected to each other (like links in a chain) in proper linear, chronological order. Every block contains a hash of the previous block. The blockchain has complete information about different user addresses and their balances right from the genesis block to the most recently completed block.
The blockchain was designed so these transactions are immutable, or forever, never to be deleted. The blocks are added through cryptography, ensuring that they remain meddle-proof: The data can be distributed, but not copied. However, the ever-growing size of the blockchain is considered by some to be a problem, creating issues of storage and synchronization.