IPAs are a style that tend to have more alcohol and more hoppiness than a lot of other beers. And, basically, with Dogfish Head's xx-Minute IPAs the higher the number of minutes on the label the higher the alcohol content and the more bitter the beer is.
Most IPAs are made from a recipe calling for more malted barley than a traditional pale ale; this means there's more sugar for the wee beasties known as yeast to convert into alcohol and bubbles.
60-Minute IPA is 6% alcohol,
90-Minute IPA is 9% alcohol and
120-Minute IPA was originally a (in brewery founder Sam Calagione's own description to me) "<BLEEP>ing ridiculous" 20% alcohol, but that's been reduced to a mere 18%. (Sam describes each of these beers in video clips at the links I've included.) A longer fermentation process is needed to reach those higher alcohol levels.
A lot of hops are used to make these beers, which is also a hallmark of the IPA style. So, a lot of hops go into 60, more hops are used for 90 and still more for 120. Hoppiness is measured by brewers in something called International Bittering Units or IBU. Anheuser-Busch's
Budweiser (this is from memory, so it may be wrong but I'm close) is 11 IBU; 60-Minute IPA is 60 IBU, 90-Minute IPA is 90 IBU and 120-Minute IPA is 120 IBU.