Blockchain is essentially the convergence of networking and databases. It allows for all machines on a network to share the same version of reality. Any changes in one place is shared across the entire blockchain. You could say it is a ledger of information wherein information cannot be changed once it is appended to the blockchain.
We often hear about the example of cryptocurrencies such as Bitcoin when people speak about blockchain, this is because cryptocurrencies are the original blockchain use-case. In this case the ledger stores all the transaction and balance details of cryptocurrencies.
Then we witnessed another evolution where self executing code was added to the ledger and this led to the 2nd generation of cryptocurrencies and smart contracts, Ethereum is an example of this.