A Release Plan is a document that provides details of all the features of an upcoming product release. Normally, a Release Plan only covers a short period of time, is tactical in nature, and is primarily for the use of the product manager and development teams.
A Release Plan should not be confused with a Product Plan. A Product Plan is involved with broader strategizing. A Release Plan tends to be more specific: it deals with resource allocation, direction and tasking of team members, phases of development, and projected milestones in the development process. Ultimately, the plan is important in providing developmental focus.