I’m borrowing the following directly from Modernist Bread, volume 3, page 10:
The basic types of commercial yeast are fresh, active dry, and instant.
Fresh yeast comes in a block form and is partially hydrated. It lasts only 2-3 weeks after opening and must be kept refrigerated. It does not stand up well to time and requires more care and attention than dry forms of yeast.
Active dry yeast is commonly available in most grocery stores for home use. Proofing will take longer when using active dry yeast. It has, ironically, the lowest amount of active yeast (by weight) of either fresh or dry varieties. Thus, more of it must be added to a recipe than other types of yeast; it also needs to be activated in warm water (or other liquid) before it is mixed into a dough. (Note from Katherine – some brands do not require “activation” any longer, and are still called active dry yeast.)
Instant yeast is our preferred version of commercial yeast; (Note from Katherine – mine too. Learned long from Jeffrey Hamelman you can store it in the freezer, lasts indefinitely.) it is more active because it has more living cells by weight than active dry yeast does. It gets its name because it is instantly ready to use and doesn’t require activation in a warm liquid as active dry yeast does.
We sell the instant yeast to all our bakers at “cost”. It’s a good deal, and you get 1 lb. Which is quite a bit to go through, but you can store it in the freezer, which is where mine is right now!
Hope that helps Hazel!