In short, the term block refers to computer files that store transaction data. These blocks are arranged in a linear sequence that forms an endless chain of blocks – hence, the term blockchain.
So all information about blockchain transactions is gathered and recorded inside these blocks, and every newly generated block is connected to the previous one through the use of cryptographic techniques.
The chain of linked blocks stores all the transaction data generated since the launch of a particular blockchain. So the records go all the way back to the first block, which is referred to as the block zero or genesis block. The number of confirmed blocks since the genesis block is denoted as the block height.
Taking the Bitcoin blockchain as an example, blocks are made up of multiple elements. Bitcoin blocks contain (among other things) a list of recent transactions, a timestamp, and a reference to the block that came right before it. This reference is a cryptographic hash of the previous block’s data.
The generation of a new block always includes the hash of the previous one, and this is what makes the blocks cryptographically linked. Such a structure allows the creation of a secure database that is highly resistant to tampering and attacks. The block hash acts like an identifier. It is unique to each block and is produced through a process called mining.
Basically speaking, the block hash is the solution to a complex mathematical problem, and the miner that finds a valid solution for the next block is granted the right to validate that block (and the transactions therein). Since mining requires a significant amount of computational resources, successfully mined blocks produce new Bitcoins as a way to reward miners for their work (see block reward).
But sometimes, two or more miners may find a valid block hash at the same time, so that two different blocks are broadcasted to the network. This leads to the creation of two competing chains. To address this problem, the participants of the network (nodes) will choose the chain that eventually becomes the longest (with more “accumulated work”). The other chain is then discarded, and its blocks become stale blocks (see orphan blocks).
Although blocks are usually discussed in the context of cryptocurrency transactions, they may also be related to other types of digital data that are stored on a blockchain system.