A radio ad is a produced audio commercial that can air on over-the-air radio stations as well as internet-based radio services like Pandora or Spotify. Radio advertising has a niche in and of itself in that radio in this day and age is typically listened to by people commuting to or at work. Many advertisers want to target that audience. The trade-off with radio is that it typically works better with repetition (running multiple commercials overtime to keep the message in the listeners' head). A radio listener simply isn’t typically in a forum where they can write down a phone number or web site right away but with repetition, it’s a medium that has proven to generate response.
Some methods to increase effectiveness include having your copy read by local radio talent and having easy to remember response mechanisms like phone numbers and web sites.