Its a new 'decentralized' blockchain network. I believe SnarkOS is the virtual machine used to run (d)apps (exchanges, games, markets). Most popular blockchains (Bitcoin, Ethereum) are psuedo-anonymous. And Aleo aims to be private by default, with the use of zero-knowledge protocols.

There are private blockchains like Monero, however they're some-what limited to just the cryptocurrency use-case.