The Backlog is a list of deliverables to be completed, like bug fixes, infrastructure changes, changes to features, and new features. They are prioritized by business need, and written as User Stories. You don’t work on backlog requirements until they’ve been given the thumbs up for development.
https://www.atlassian.com/agile/project-management/user-stories
The Backlog is the single source for activities that a team needs to work on. Nothing gets done that isn’t backlog before it gets approved for production or development. Items in the backlog are not greenlighted yet, but there for discussion and prioritization before moving them out of the backlog and into development.
User Stories are the most common format for items in the backlog.
Story Maps and Information Radiators provide a clear picture of the Backlog for the project team and stakeholders.
The Backlog will often be represented using index cards, sticky notes, Microsoft Excel, or software like Jira or ServiceNow.
Sprint Backlogs are a different concept, but are related to Product Backlogs.
Scaled Agile Framework (SAFe) uses a series of backlogs that replaces the Product Backlog. Each type of Backlog contains a description of backlog items based on a backlog item hierarchy.
From Agile Alliance: https://www.agilealliance.org/glossary/backlog/
1. The portfolio backlog contains the different initiatives an organization is considering (referred to as epics).
2. The solution backlog contains high level backlog items (referred to as capabilities and enablers) that represent aspects of a solution
3. The program backlog contains backlog items (referred to as features) that represent aspects of a solution.
4. The team backlog contains backlog items (user stories and others) that a team works on.