According to Criminal Defense Lawyer.com, a class D felony is a subset of the felony category which means that it's still a serious crime, but it's not quite as serious as a class A or B felony. Federal and state governments separate crimes into major crimes, or felonies, and minor crimes, or misdemeanors. Some states use a further classification to determine the severity of its felonies.
The classification of felonies varies by state and will depend on the state's penal code and administration's view on the crime. What one state considers a class D felony, another state might consider a class C or a class E felony. Some examples of general class D felonies include unlawful surveillance, prohibited use of a weapon and criminal sale of a firearm with the aid of a minor. However, each state may classify these examples differently.