Backlog items can be anything, not just requirements. They can be tasks, defects, planned work, risk items, and much more.

It is important to know that items in the backlog are prioritized relative to the others. 

The level of detail used to describe a backlog item can vary. Most of the time, backlog items at the top of the list are described in more detail than other items.

Backlog items are reviewed in terms of available capacity, dependencies, the size of the item, and relative complexity.