A RACI Matrix describes stakeholders role in the given task or deliverable.  The roles are broken down into the following four categories and stakeholders may have more than one role.

[R] for Responsible - this is who does the work.

[A] for Accountable - who is the decision maker.  Typically there is only one of these per task or deliverable.

[C] for Consulted - identifes who must be consulted prior to the work and gives input

[I] for Informed - mean these people must be notified of the outcome of the task or deliverable.

RACI Matrices are useful tools and techniques for quickly and clearly communicating roles and responsibilities to a team. They are fairly simple to do, yet its surprising the number of discussions among a team that arise when these are done as roles are clarified. 

Just think about the value added to a project as the duplicate team member work or missed work is uncovered through the use of such a straightforward technique.