The blockchain, also known as distributed ledger technology (DLT), keeps a record of cryptographic transactions made in peer-to-peer networks. The data recorded is organized into blocks in chronological order and secured by cryptography.
When a new block is created, it takes data from the previous block, creating a link – hence the term blockchain. In a blockchain, if any data in any block is altered, the entire blockchain from that point onwards is broken. You can think of it like a tower of wooden blocks, if you break one block in the middle of the tower, all the blocks above it topple over. The only way to 'fix' the tower is by correcting the data that was tampered with. In fact, blockchains are often measured in terms of 'height' which is the total number of blocks in the tower. So, the older the data is, the more secure it becomes. Generally, a block will be considered 'valid' once enough additional blocks are added to the chain in order to ensure security.
In cryptocurrencies, the blockchain is used to store an immutable transaction ledger for said currency. Currently, blockchain technology is primarily used to record cryptocurrency transactions, but it can be used for several applications. The earliest and largest blockchain network is Bitcoin.