It is a chain of blocks. Blocks are encrypted data. The generation of blocks (the encryption) is done by miners. They compete to find the first block that respect the rules of the blockchain. They are rewarded for this. Miners are all over the world, so the system is distributed and thus not easy to take down. A blockchain has an application, like currency. Bitcoin is the most famous example. The blocks are the bitcoin transactions, from and to (to a certain degree) anonymous adresses. They are encrypted with SHA256 algortihm and added to the chain, the ledger. The ledger contains all the transactions from the very first one to the current ones. Copies of the ledger are spread over the world, every one can download it. Due to the improvement in mining speed and efficiency, the encryption gets harder and harder to compensate fo the advance in mining rigs. Anyone wanting to change a transaction of the past (steal bitcoins and transfer them to himself) needs to generate all blocks until the most recent one, will he ever be able to replace the ledger (if even possible since the ledger is diustributed). Anyway, important for a blockchain is encryption and distribution, it is a concept without middle man (like banks in case of currency), without fraud and decentralised. Bitcoin is an application. And many more applications can be implemented with blockchains. Nowadays many try to launch the next blockchain application (or platform for applications on blockchain) to find the holy grail. But the hype attaracts also a lot of starters with less noble intentions. But blockchain has a lot of potential and will be important wihtout any doubt.
What is blockchain? How will it affect us?