Baker's yeast

There are several types of Baker's yeast:

Cake Yeast, also known as wet, fresh, or compressed yeast, is characterized by a high moisture content. Cake yeast is processed one step further than cream yeast and is taken directly from the fermentation vat. the yeast cells are alive and produce more leaving gas than other forms. It is very perishable and needs to be refrigerated. Shelf life is short, between 2 and 8 weeks.
Active Dry Yeast is yeast is processed one step further than Cake Yeast. It is taken from the fermentation tank and dried into granules with a protective coating of yeast debris. Due to the low moisture content, the yeast is in a semi-dormant state and is therefore more stable than cake yeast.  At the time of usage, the baker needs to reactivate the yeast by soaking them in warm water, 105-110 degrees F/41-43 degrees C, before adding mixing into the dough. The shelf life of an unopened package extends from a few months up to 2 years.

Instant Yeast, also known as "fast-rising" or "fast-acting" yeast, is a "dry" yeast that can shorten the rising time in traditional baking by as much as 50%. This yeast is dried quickly and into porous rods instead of granules. This rod formation allows them to take up water more rapidly. Instant yeast does not need to be prehydrated and usually produces more carbon dioxide gas than active dry yeast The shelf life of an unopened package extends up to 2 years.