RACI is an acronym that stands the four key responsibilities: Responsible, Accountable, Consulted, and Informed; also know as responsibility assignment matrix. Its a formal way for establishing role for each stakeholder especially when multiple parties or processes are involved.

Responsibilties:

1. Responsible: The person who is assigned to do the work.
2. Accountable: The person who has decision authority and has ultimate ownership for the work. As a golden rule, there should be only one accountable for each task.
3. Consulted: The person who is domain experts and must be consulted before taking any decision/action. Communication flows both ways since these person knows and they share information.
4. Informed: The person who needs to know that a decision or action has been taken. Communication flow is one way only and they need to be kept informed well.

Consider below sample RACI matrix for clarification purpose:

Hope that was pretty simple!!