It's great that you're thinking about both backup and storage--the two are definitely not the same.

Storage
If you need increased storage capacity and want to be able to access the stored data easily, you should look into adding an external hard drive. If you have multiple users accessing the same data, check out a networked storage drive.

Backup
On the other hand, a hard drive can fail and take all of your important data with it. For backup, look into DVD burners. This will allow you to burn your valuable data onto DVDs and store it in a separate location. That way, if anything happens to your computers and drives, you'll have the data safely squirreled away. Good luck!