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Yeast
Yeast is a crucial element in the conversion of flour to many sorts of leavened bread. Yeast is a living, microscopic, single-cell organism that, as it feeds, converts sugars and starches into alcohol and Carbon dioxide. This process is known as fermentation. The trapped carbon dioxide gas is what causes dough to rise. Without yeast there is no fermentation, no production of Carbon dioxide and the bread will not rise.
However there are many different sorts of yeast such as: active dried, inactive dried and compressed.
ACTIVE DRY YEAST (ADY)
A less common type of yeast that before combining with other dough ingredients, is activated by re-hydration, or "proofing," in warm but not hot, water. Some brands of active dry yeast can also be pre-combined with a measure of the flour and other dry ingredients. The advantages of active dry yeast include convenience of use and long storage life, especially when sealed or frozen in an airtight container.
INSTANT DRY YEAST
Perhaps the commonest and most readily available type of yeast. This is a dried yeast, typically in small beads or tiny pellets that can be added directly to a dough mix or in a bread mixer with no need for prior re-hydration. Like ADY it has a long shelf life especially if kept in an airtight container in a fridge.
FRESH or COMPRESSED YEAST
This form of yeast is essentially the freshest form of commercially available yeast. It has a much shorter shelf life even if kept chilled. It is typically sold in small blocks and is grey and crumbly in texture. It usually only keeps for 2-3 weeks and when it becomes slightly brown and "plasticy" it should be thrown away.
How much to use
Usually when following a recipe you will be told how much to use. Make sure that you are clear whether the recipe is referring to dried or active or fresh yeast as you will need different amounts accordingly.
Typically, the faster you want the bread to rise the more yeast you use. You can make fantastic bread with a very small amount of yeast if you are able to leave dough to prove for 24 hours for example. 1-2g as opposed to 30g for a short 2 hour ferment.
If the recipe says FRESH yeast and you only have dried then use 2/3 of the amount of dried by weight. Similarly if you only have fresh and it says use DRIED then use 1/3 more.