[mattkrause]

The restrictions @mkmk is talking about apply to public radio stations, like NPR (and maybe some college stations).

The key distinctions are that underwriting has to be short (15 seconds each, 60 seconds total), non-competitive (you have to mention them all underwriters of a program), only at the beginning/end, and can't include a call to action. In other words, an underwriting message can say "This program brought to you by Crazy Eddie's Autos, purveyors of new and antique European cars" but not "COME ON DOWN to CRAZY EDDIE'S AUTO'S. WE'RE HAVING A BLOW-OUT SALE THIS WEEKEND ONLY blah blah"

Privately-owned stations can and do run either kind, however.