ABR files are brushes, ie brush files of the graphics software Adobe Photoshop. An ABR file contains individual or collections of different brushes. A brush includes shapes, textures and patterns. Adobe Photoshop is the standard program for professional image editing.
Photoshop brush files are usually placed under / Presets / Brushes in the program folder. Abr files can be loaded and saved via the menu of the brush palette. The ABR format is a proprietary format and can only be used by Photoshop. Using the Windows program abr2gbr, however, you can convert Photoshoppinsel into the GIMP brush format gbr. (with restrictions)
Alternatively, abrviewer.NET ABR files (also current version CS5) can be converted into png files using the free Windows program. Each included brush becomes a png file.