Fresh (cake) yeast used to be the only kind available, but it's extremely difficult to find nowadays. I know it's difficult for people like us to comprehend, but there just isn't a demand for yeast of any kind. Mainstream grocers prefer to carry dry yeast with its longer shelf life than fresh yeast, which not only has a shorter life, it also needs to be refrigerated (better to use that electricity on something with a higher profit margin, like, say, Kraft Philadelphia cream cheese). (Sorry for the sermon.)

There are only three types of yeast: fresh, active dry, and instant. Pay attention to these types of yeast, not to their brand names.

I am bound to invoke the wrath of other bakers by saying that there is no difference in taste (or looks or height) between a loaf of bread made with a cake of yeast and a loaf of bread made with a packet of dry yeast. They can come at me with all the batards and baguettes they can muster--I don't care because I've got Peter Reinhart ("The Bread Baker's Apprentice"), the San Francisco Baking Institute and Cook's Illustrated behind me. This is how the folks at CI explain it:

"Fresh Yeast is about 70 percent water by weight and is composed of 100 percent living cells. It is soft and crumbly and requires no proofing—fresh yeast will dissolve if it is simply rubbed into sugar or dropped into warm liquid. Owing to qualities associated with its strain, fresh yeast will produce the most carbon dioxide of all three types of yeasts during fermentation. Fresh yeast is considered fast, potent, and reliable, but it has a drawback: it is highly perishable.

Active dry yeasts arrive at their granular state by undergoing processes that reduce them to 95 percent dry matter. Traditional active dry yeast is exposed to heat so high that many of its cells are destroyed in the process. Because the spent outer cells encapsulate living centers, active dry yeast must first be dissolved in a relatively hot liquid (proofed) to slough off dead cells and reach the living centers.

Also called "Instant" or "Rapid Rise," or "Bread Machine," instant yeasts are also processed to 95 percent dry matter, but are subjected to a gentler drying process than active dry. As a result, every dried particle is living, or active. This means the yeast can be mixed directly with recipe ingredients without first being dissolved in water or proofed. It is in this context that the yeast is characterized as "instant." We prefer instant yeast in the test kitchen. It combines the potency of fresh yeast with the convenience of active dry, and it is considered by some to have a cleaner flavor than active dry because it contains no dead cells. (In our months of testing, we found this to be true when we made a lean baguette dough but could detect no difference in flavor when using the two yeasts in doughs made with milk, sugar, and butter.)"

I keep a 1 lb. bag of active dry yeast in my freezer at all times; I use two, almost three bags a year, mostly for our once-a-week pizzas and the six loaves of raisin bread I bake every other week. I also keep a jar of instant yeast in my refrigerator door, which I use for quick one-rise dinner rolls. Two daughters-in-law have bread machines and purchase yeast by the jar, which they keep in their cupboards.

If you have a recipe that calls for fresh yeast but you have only dry: one cake (0.6 ounce) of compressed fresh yeast = 1 packet (.25 ounces) of active dry yeast.

I think fresh yeast (by which I assume you mean refrigerated cake) makes the best tasting bread - though I'd probably flunk a definitive taste test. But it is nearly impossible to find at least here in New England - the stores say they sell too little, and it has a short self life. Active dry works just fine and so does instant (which you mix with dry ingredients - don't proof it). They are also dehydrated and last much longer - esp. if refrigerated. Google them to get the permutations. Yeast is yeast, but they are processed differently and added to ingredients in different ways.

By fresh active yeast, are you referring to cake yeast or just the difference between the brands? I think you mean the former, so I'll answer that.

You're going to have a difficult time finding fresh yeast. I haven't seen it sold it years. You'll find it in the refrigerator case, while the active dry yeast will be in small packages in the baking section. There's really no difference. Each has to be proofed in a liquid. Use the same amount of liquid and just substitute one for the other.

Don't confuse either with instant yeast. That's just added to the dry ingredients.