Northwest has this explanation:

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What is a Promo Fare?
From time to time, Northwest Airlines, offers promotional fares on a limited basis. nwa.com Reservations will automatically advise you of the promotional fare. As you review the list of fares, you will see a star and the phrase "Promo Fare" on flights where a promotional offer is available. Review the flights carefully; the star with `Promo Fare' next to it indicates that at least one segment of your trip qualifies for the promotional fare.
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But that doesn't really explain what a promo fare is, or why I should "review the flights carefully" if I see a star indicating a promo fare.

Are there more restrictions on a promo fare (vs any other fare)? Are promo fares the first to get bumped if a flight is overbooked?