Leavening is a generic term that refers to agents that cause the dough to rise, while yeast is a type of leavening.  Yeast is available in three forms: fresh, active dry, and instant.  Fresh yeast is also known as cake yeast or compressed yeast. It's the most perishable form of yeast and must be used within a week of purchase.  Active dry yeast is the most typical form of yeast sold in supermarkets. It's made by dehydrating fresh yeast, which makes it more shelf-stable.   However, active dry yeast must be dissolved in water before using it. Instant yeast is also known as rapid-rise or fast-acting yeast.  It is not necessary to dissolve it in water before using it; instead, it can be put immediately into the flour. Fresh or active dry yeast has a shorter shelf life than instant yeast.