Yeast (also called "brewer's yeast" or "baker's yeast") is a living product, made up of microscopic fungi which grow slowly when they are in a warm place.
Baker's yeast is for use in breads and viennoiseries. It it is incorporated gently into the dough, then you need to wait for it to work.
It should not be confused with baking powder, which is used for raising cakes.
If you intend to buy:
Yeast is available in two basic kinds: fresh yeast (ask your baker to sell or give you some, it looks like a greyish paste, which kept in a sealed box in the fridge and should be used within a few days), or dried yeast in 5 g sachets (on left on the photo). Dried yeast will kep for several months without any problem. It can be used like fresh yeast, but only half the quantity is needed (it's a very effective yeast).
Personally, I frequently practice: 1 dried yeast sachet = 10g fresh yeast, and vice versa.Note
: For each recipe you can use either fresh or dried yeast, use whichever you have to hand.
If you want to use it: Yeast is a delicate product, it doesn't like cold (which slows its action), direct contact with sugar (which can kill the yeast by "burning" it), and excessive heat, but contrarily to a very comon idea, direct contact with salt don't kill it.