Generally, it's possible to be convicted of a DUI as a misdemeanor or a felony. A standard first offense is almost always going to be a misdemeanor. But a DUI offender who kills or seriously injures another person is typically looking at felony charges—even if it's the person's first offense. (Read more about criminal charges for DUI-related killings.)
Having prior DUI convictions can also elevate a DUI to a felony. In some states, first and second DUI offenses are misdemeanors but a third or subsequent conviction is a felony. (Learn about how long a prior DUI conviction stays on your record.)
There are also states that make a DUI a felony if the driver had a particularly high blood alcohol concentration (BAC) or was transporting children while driving under the influence.
(Check out the DUI/DWI laws in your state.)