What is Yeast?
Yeast are very small, single cell organisms. Yeast are everywhere, on plants, in soil, and in the home. One species of yeast, Saccharomyces Cerevisiae, has been "domesticated" over the centuries to produce good beer. Now there are hundreds of different strains of Saccharomyces Cerevisiae available to make beer.

What Makes Liquid Brewers Yeast Different?
Liquid yeast has many advantages over dried yeast. The variety of liquid strains is much greater, but most important is the flavor profile. Most brewers would agree that beer made with liquid yeast is superior in flavor, and consistently wins medals over dried yeast in national competitions. Most liquid yeast on the market is supplied in small quantities. To make homebrewing easier, White Labs produces vials of liquid yeast that are ready to pitch into 5 gallons of beer. Each container's cell-count corresponds to a pint size starter, which saves the brewer 2-3 days and $1.50-3.00 in material cost. If more than a pint starter is desired for pitching (for beers over 1.070 Original Gravity, cold fermented lagers, or homebrewers wanting a faster fermentation), a 1-2 liter starter can be made in just one day.

How is Yeast Used?
Once wort is transferred into the fermentor, shake the fermentor vigorously to get plenty of oxygen into solution. This will help fermentation, and is the only time oxygen should be introduced into the brewing process. Brewers call adding the yeast "pitching". Before pitching, make sure the wort temperature is between 62-72 degrees F. Too cold and the yeast will take too long to begin fermentation, too hot and the yeast can be killed.  Allow the yeast to get to room temperature to prevent a temperature shock. Shake the vial to dislodge all the yeast, open cap and pitch! Shake fermentor well to mix up and aerate.

What Should the Fermentation Profile Look Like?
Normal fermentation will begin 5-15 hours after pitching the yeast. During this "lag phase", yeast become acclimated to their environment and uptake sugars and oxygen they will need for the fermentation. The first sign of fermentation activity will be a raised airlock. This signals CO2 production. A fine layer of foam will then form on top of the liquid. Within a few hours the head will get rocky and the airlock will quickly expel CO2. Fermentation will usually be complete in 5-20 days. If there is not enough room for foam, the foam may get into the airlock and then blow the airlock off the container! So be careful and allow plenty of headspace, or use a blowoff tube for the first 3 days. The airlock will bubble very slowly, and the yeast will begin to drop to the bottom of the fermentor. Check the gravity, if fermentation is complete and yeast is still on top of the beer, either cool the fermentor to force the yeast down or transfer the beer into a different container. Now the beer is ready to keg or bottle!

Source: White Labs Website