Not a silly question at all. There is essentially no difference. Both can perform the same job equally. I believe DMD (doctor of dental medicine) was used originally, someone correct me if I'm wrong. But, somewhere along the line someone thought dentistry was more of a surgical specialty, and dubbed their grads DDS (doctor of dental surgery). A lot of schools followed their lead and did the same. So, today we have both. If you'll notice, most of the older schools use DMD (Harvard, etc.) while the newest schools use DDS.
Feel free to ask any other questions that may seem silly, and I will do my best answer them.
