Yeast is the ingredient in beer that does all of the hard work. It converts the sweet wort into alcohol and gives off CO2 gas. This process is called fermentation. Yeast is very unstable and needs to be treat with care. Any contamination by 'bad' bacteria can cause the yeast to die or produce off flavours. It is of the upmost importance that the FV is cleaned and sanitized using correct cleansing agents. It is also extremely important that the yeast does not come into contact with these agents. If it does then it can die and you wont have any beer... and we wouldn't want that.
Below are 2 examples of dry brewers yeasts. These are very popular with homebrewers due to the length of time they can be kept. They are also very easy to use. They can be simply sprinkled onto the top of the wort or rehydtrated in a luke warm glass of water 10-15 mins before pitching. Due to the processes required to make dry yeasts, the final quality of the yeast is affected. However, due to recent advances in technology this is becoming less of an issue and there are many high quality dry yeasts available.
Another type is liquid yeasts which are generally available in a small vial. These vials contain yeasts which are generally of a better quality than dry yeasts due to the fact that they are not processed in the same way. One of these vials does not contain enough yeast cells to ferment your beer properly, so before pitching, a yeast starter must be made. A yeast starter is used to cultivate more yeast cells before pitching. It is usually made 2-3 days in advance and can be made from luke warm water and spray malt. The amount of yeast cells then increase allowing the starter to be pitched directly into the wort. More on making a yeast starter is available in the methods section.