Are you familiar with Napster or Bit Torrent which is a form of peer to peer file sharing? Basically, if I have a file, I can share that file in a network of computers and everyone will download it from me and have a copy of that file. At a high level, Ethereum is similar but we are all sharing an Operating System like Windows or Mac, and it is constantly being updated and checked by all the computers within the network. It is so complex that we can even run programs on it, like payment processor, a chat application, or even a Facebook.
The difference between Facebook/Google/Whatsapp and applications on Ethereum is that Facebook are still centralized. Since it is a centralized these companies dictate user terms and conditions, policy, privacy, and other things. Ethereum provides an decentralized alternative to this and "in theory" giving the power back to the users and making it more democratic.