Photoshop brush (ABR) is a file format used by Adobe Photoshop for brushes (graphics, and information about brush behavior).
Note that the information in these documents might not apply to all versions of the format. The current version does not appear to be well-documented.
- Photoshop File Formats Specification V6.0 Release 2
- Photoshop Brushes (ABR) Specification (from archive.org)