since a lot of replies branched towards Cryptocurrency, which is where blockchains are implemented the most in. But it isn't the sole purpose of blockchain.

It's a distributed, append only(theoretically), tamper proof data structure. Look up merkle tree, certificate distribution, etc. These comes in different shapes and sizes for storing transaction logs, to keeping track of online identity and false impersonation.

You can implement a blockchain that might not get as power hungry as crypto block chains(because mining), and it's a cool solution in distributed systems