You have both asked for some basic information on yeast, so let me see if I
can help.
Active dry yeast is the yeast that comes in the little envelopes in the
supermarket. It works fine, but if you bake a lot of bread, it gets expensive.
Instant yeast is usually bought in larger quantities, and often comes in a
vacuum-sealed "brick" or in a jar (sometimes labeled bread machine yeast).
They measure virtually the same, teaspoon for teaspoon -- my experience is
that the difference is so small, it is not worth trying to make any
adjustments. One packet of active dry yeast equals about 2 1/4 to 2 1/2
teaspoons of instant yeast.
They can be used interchangeably in recipes, with the following change:
Active dry yeast must be rehydrated in water before you use it (also called
proofing). The process is simple -- you mix the yeast with about 1/4 cup
of the liquid from your recipe, warm to the touch but not hot. Let it sit
for about 5 minutes (if you are concerned that your yeast is no longer
good, add a pinch of sugar; if the mixture bubbles after about 5 minutes,
the yeast is good). Then mix the yeast mixture in with the rest of the
liquid in the recipe (be careful that the other liquid, if it has been
heated, is allowed to cool to "warm but not hot" or you risk killing the
yeast). Instant yeast does not require rehydration, so you can just mix it
in with your dry ingredients and proceed as usual. Because it does not
require mixing with water first, instant yeast works better in a bread
machine, especially if there is a delay between when you add all the
ingredients and when the machine starts.
As for storing instant yeast, I keep my unopened packages in the pantry;
once open, I put them in a zipper bag in the refrigerator. Some people put
it in the freezer, but I have not had good luck with that. I find it keeps
fine in the fridge for at least 6 - 12 months.
Instant yeast can be purchased at some supermarkets (the bottles of "bread
machine yeast"), in catalogs and specialty stores (the good old King Arthur
catalog comes to mind) and, I have heard, occasionally from bakeries.
Do not confuse instant yeast with "Rapid Rise Yeast," which also comes in
envelopes in the grocery store; this is a yeast that is formulated for
making bread very quickly. Some people like it, but my view is that bread
made this quickly is not very good.
Hope this helps. Keep on baking!
Barbara