Yeah the "minutes" is the duration the hops are added to the wort during the boil. That is the fundamental difference. However, the 120 uses a bunch more barley in the mashing process(producing a lot more fermentable sugars) than say the 60 or the 90 min. It's necessary in order to balance out all the hops, and ofc it produces a bunch more alcohol during the fermentation process.
The DFH 60 min IPA is 6% ABV, the 90 min is 9%, and the 120 is a whopping 16-18%, depending on the batch. So as for taste, the 60 is mild and somewhat hoppy, the 90 is more malty, and the 120 is smooth, a tad sweet, but boozy, with the insane amount of barley(malt) balancing the 2 hours of hopping during the boil. The 120 is also dry hopped, which adds additional hops after the boil, mostly for aroma.