A backlog on an Agile project is a collection of user stories, features and technical tasks that a team maintains. The backlog contains a prioritized record of all of the work understood to be required to complete a project. The backlog takes many forms; it may be housed within a tool designed to manage agile projects, in a spreadsheet or on sticky notes on the wall of a team room.
There are four types of backlogs: Portfolio, Program, Release and Iteration. Some or all of them may be used to manage an Agile project depending on the size and scope of the effort.
Portfolio
The portfolio level of a backlog typically houses epics, or very large stories that represent work to be done across multiple project teams toward a common goal. Often these epics represent cross-cutting work that serves multiple programs across a portfolio. They are reviewed frequently, prioritized and remain in the backlog awaiting scheduling and implementation at the program, release and then iteration levels.
Program
The program backlog represents the work from the portfolio backlog that has been approved for implementation by a project team. Moving items into the program backlog signifies that they are ready to be decomposed, estimated and scheduled in a near-term release.
Release
The goal of a given release is to deliver a subset of the program backlog. Typically, work is chosen from the program backlog in such a way that a “meaningful” piece of the product can be built during the release time frame.
Iteration
Within each release there may be multiple iterations that bring the team closer to their release goals. An iteration (sometimes called a “sprint”) backlog represents work that is currently underway.