I did some of my own research on this, and found this useful link:
Follow these instructions to install brush sets:
1. Download or move the .ABR file to a permanent location on your computer. This could be into the Photoshop Brushes folder (full paths below) or a central location you create for Photoshop brushes. To place it in Photoshop’s existing brush folder, move or copy the .ABR file to:
Windows: C:\Program Files\Adobe\Adobe Photoshop CSx\Presets\Brushes\
Mac OSX: Applications/Adobe Photoshop CSx/Presets/Brushes/
2. Open Photoshop and select the Brush tool.
3. On the Options bar, click the down arrow beside the brush shape preview. This will open the controls for brushes.
4. From the brush control popup, click the right-facing arrow in the top right corner and select Load Brushes or Replace Brushes.
5. When the Load Brushes dialog appears, navigate to the location where you placed the .ABR file and then open that file.
6. If you chose the Replace Brushes command, Photoshop will remove any existing brushes from the Brushes palette, leaving you with only the newly loaded set. However, if you chose the Load Brushes command, Photoshop will add the new brush set to the end of the list of brushes.
7. Start brushing!
Unless you specifically use Finder to move an ABR file to the Applications/Photoshop/Presets/Brushes folder, only stock items will be there. When you save your own ABR files, the default location that they will want to be stored is in your user folder: Users/[Your user]/Library/Application Support/Adobe/Adobe Photoshop [Version]/Presets/Brushes, although you can save the files anywhere. If you have loaded brushes into Photoshop, they are not stored anywhere as an ABR file. Instead, they are part of your preferences.
If you are looking to back things up, you will want to save the brush sets out (Preset Manager will probably make this easiest for you) and create new ABR files (if you no longer have the ones you downloaded to install them in the first place).