A human organization can be defined as combination of two things: a set of property, and a protocol for a set of individuals.
A decentralized organization (DO) takes the same concept of an organization, and decentralizes it. Instead of a hierarchical structure managed by a set of humans interacting in person and controlling property via the legal system, a decentralized organization involves a set of humans interacting with each other according to a protocol.
A decentralized autonomous organization (DAO) is an entity that lives "on the internet" and exists autonomously, but also heavily relies on hiring individuals to perform certain tasks that the automaton itself cannot do.
Another definition of DAO is available here: a decentralized autonomous organization (DAO) is an entity that exists in multiple redundant locations, has sovereign control over internal capital (assets), and prohibits unintended modifications of its assets or mechanisms. A decentralized autonomous organization performs the role of manager of human behavior within an organization.
Assets: internal currency, tokens, etc;
Actors: curators, moderators, token holders, robots, smart contracts, etc;
Rules of governance/protocol: permissions, business logic, etc.