Cryptocurrencies are built on computer networks. They make use of an underlying data structure called a blockchain. A blockchain, simply put, is a decentralized digital ledger. It consists of a collection of blocks or nodes that maintain transactions taking place within the blockchain across multiple computers. They also contain a link to their previous block along with a timestamp. This ensures that none of the blocks can be modified, without the alteration of all subsequent blocks. For this reason, blockchains are very secure.