In many states, a first time DUI counts as a misdemeanor. Although the punishment for a misdemeanor is no walk in the park, it is much more preferable to receiving a felony DUI.
If, while driving under the influence, you should happen to injure or kill another person, you can be found guilty of a felony. You may then end up going to state prison for several years if convicted. Having prior DUI arrests can also result in felony charges or harsher sentences than for first-time offenders.
If you happen to be in a car accident, make sure that you consult with qualified car accident attorneys.
In some states, multiple DUI convictions are enough to warrant a felony charge. This is regardless of whether or not you harmed another person.