The D25 is very similar to our current D45, which you can read all about on our website. The main differences are that the D45 is a little bit larger, so that it can accommodate dampers, and it has an extra course of strings at the top of each bridge. Also, it's currently made with a sapele soundboard and back, while your D25 is probably mahogany. Did you know that Rich Mullins played a D25?
If it didn't come with the dulcimer, you might find our Dulcimer Owner's Guidebook to be a handy reference. You can find it on our Hammered Dulcimer Support page, along with some other helpful tips about things like tuning and general maintenance.
Have fun playing!