The concept of DAO (Decentralized Autonomous Organization) has been known since 2013, when the co-founder of EOS, Dan Larimer, first mentioned the term. Currently DAO has become one of the most innovative Blockchain-based concepts. The main philosophy of DAO is to create an organization without the centralized control.
Simply DAO is a smart contract, where all rules are written in the source code instead of the piece of paper. All decisions in DAO are made in a completely democratic manner by the vote of its shareholders according to the previously established rules. These rules are adopted by all participants, at the moment of DAO creation. In other words, DAO does not have top-down hierarchy, there is no centralized power.