Active Dry Yeast: This is the yeast called for in most bread recipes. Granules are dryer than fresh yeast and therefore have a longer shelf life‚Äîabout 4 to 6 months in the refrigerator. It is often dissolved in warm liquids to "start" it before mixing with other ingredients. Packets of active dry yeast are 2 1/4 teaspoons (1/4 ounce) and are usually sold in the dairy case at grocery stores.
Quick Rise/Fast Rising/RapidRise/Instant Yeast: a more active strain of dry yeast with smaller granules, it allows you to make bread with just one rise. It is usually sold in 1/4-ounce or 2 2/4 teaspoon packages.
Active Fresh/Compressed/ Cake/Yeast: While it goes by many names, this yeast is generally sold as a moist .6-ounce cake that readily crumbles. It keeps in the refrigerator about 2 weeks or in the freezer for about 2 months. It is more popular with commercial than home bakers.
For more information on "starting" or proofing yeast, watch our video How to Proof Yeast.