Backlog is ever-evolving list of product requirements, prioritized by the customer (or customer representative), that conveys to an Agile team which features to implement first.
Taken from agiledictionary.com
Backlog An ever-evolving list of product requirements, prioritized by the customer (or customer representative), that conveys to an Agile team which features to implement first. Agile projects typically employ a top level backlog, known as a product backlog or release backlog, and each Agile team working on a project typically creates a backlog for each development iteration, known as an iteration backlog or sprint backlog.