The three most common types of commercial yeast are:
Fresh yeast: Truth be told, fresh yeast, which is also known as cake yeast, is hard for home bakers to find these days. They are sold as little “cakes” that must be kept refrigerated, and they go bad after a few days. Many professional bakeries, however, still use fresh yeast, and so many bread formulas are written with fresh yeast in mind. Typically, breads that rise for 1.5 to 2 hours in bulk and 1 to 1.5 hours shaped call for fresh yeast at 2% of total flour weight.
Many recipes use much, much less yeast than that, however, and they ferment for much longer, which gives the bread more flavor.
Active dry yeast: This is the kind of yeast that you’re most likely to find in your grocery store. Typically, you’ll want to use about 1 to 2 tsp per loaf and, if the formula calls for fresh yeast, you’ll need to measure out 40% of that weight to convert to active dry.
Active dry yeast needs to be “proofed” before using, which means it needs to be dissolved in about ¼ to ½ cup of lukewarm water (about 90 degrees F or so).
Instant yeast: This is what many amateur bakers prefer to use. In grocery stores you'll often find it labels "Rapid Rise Yeast" or "Bread Machine Yeast." It looks like active dry yeast, but it retains many more living yeasts in each grain. As a result, there’s no need to proof it – just add the yeast directly to the dry ingredients. If the formula calls for fresh yeast, measure out 1/3 of that weight for instant yeast. If the recipe calls for active dry yeast, cut the measurement by about 25 percent.