A DB or .deb file is a kind of package that contains all the information you need to install an application to a jailbroken iPhone or iPod. Often downloaded for Cydia, once you have jailbroken your iPhone or iPod you can download these files from Internet sites or iPhone specific application stores. It is fairly easy to open and run DB files using a couple of different iPhone applications as long as your phone is Jailbroken.
Copy the DB (.deb) file onto the iPhone making sure the .deb file is in the root (main) directory of the iPhone.
Install Cydia. Cydia is a program you can install onto a jailbroken iPhone or iPod. It allows users to download any applications that are not availible for download from the iPhone App Store. For download location see resources.
Open the Cydia program. Go to "Search" and type Mobile Terminal. Download and install the program onto your phone.
Reopen the Cydia program. Go to "Search" and type OpenSSH. Download and install the program onto your phone.
Start Mobile Terminal by selecting it from your app drawer. Type "su" and press return. Type "alpine" and press return. Type "dpkg -i xxxpackagename.deb", replacing xxxpackagename with the name of the .deb file you wish to run. Your iPhone will now run the .deb file you downloaded onto your phone.
You may need to reboot your iPhone or iPod touch for the program to take effect once installed.