“The Blockchain is an incorruptible digital ledger of economic transactions that can be programmed to record not just financial transactions but virtually everything of value.”
Don & Alex Tapscott, authors Blockchain Revolution (2016)
To put it in an even more simple way, blockchain is like a distributed database, with copies of itself being hosted on a large amount of computers (millions of them) … and this database is continuously reconciled. As you can imagine, it's virtually impossible to make a change simultaneously to all of the copies at the same time, that's why it's pretty much impossible to hack a blockchain … at least with current technology.
For the sake of easy understanding, let's say that every Crypto Currency “runs” on it's own Block Chain OR uses the Block Chain of some other Crypto Currency, that is designed for such purposes.