"Ether" is a type of money that is "decentralized" which means that there's no company, bank or government that oversees it. Instead, it uses the "Ethereum" network of computers spread around the world, which keep track of every payment of ether. As in other cryptocurrencies, Ethereum keeps track of payments by using a blockchain, which is a growing registry of every payment. This blockchain is open for everyone to read, and it is spread across the network so that everyone can check what payments have been made. This also protects against hacking or theft of money from the system, because if the blockchain was to be altered in some computers then it would be noticed that it is not identical with the others.