There are 3 basic types of yeast available; active dry, instant and fresh.
Of the three Instant is the most potent. I use about 25% less instant yeast
in recipes calling for active dry, or just dry, yeast. If the recipe calls
for instant yeast and you have Active Dry yeast, try adding 25% less than
what is called for. Fresh yeast is the hardest to come by and the "weakest"
of the three. You will typically need twice the weight of fresh yeast to
equal the leavening of a stated quantity of active dry yeast.Instant yeast
generally does not need to be proofed and can be mixed with the other dry
ingredients. I believe Fleischman's RapidRise is an insant yeast and is
usually available in supermarkets.
Hope this helps,
KyleW