At its core, the blockchain is a ledger — a series of records and data points, like a spreadsheet or old accounting book.
In contrast to traditional record-keeping methods like spreadsheets or dusty old books, blockchain data is distributed across a network of computers all over the world. Therefore, there isn't a single source that can be misplaced, changed, or rendered obsolete. Instead, the blocks (or bundles of transactions) are checked at various validator nodes to ensure accuracy.
The term "block" is used to describe the practice of storing any data and the state of the network at any the time of recording the data in discrete units or blocks. For eg., If you want to send a crypto coin or token to someone else, the transaction data needs to be added to a block to be successful.
The term "chain" comes from each block containing the fact that each block contains a cryptographic reference to the block that came before it. It's like a chain reaction for blocks. Altering the information contained in a block necessitates reworking all subsequent blocks, which in turn requires the agreement of every node in the network.
Each “node” in the network must confirm the new block and the entire chain. The collective name for these computers is "nodes." Nodes ensure everyone interacting with the blockchain has the same data. Blockchains require a consensus mechanism to achieve this decentralized agreement.
There are many different blockchain technologies in use currently like Bitcoin, Ethereum, Tron, Solana, etc. each with its unique mechanism to achieve consensus and use the blockchain technology to store data, value, or even run programs.