The main ingredients needed in brewing beer are malt extract, yeast, hops and water. Other ingredients that may be included but are not essential are priming sugar, a grain bag, and speciality grains and powders.
Malt extract comes in either dry or liquid form. The main difference between the two is the amount of water left in the end product. This ingredient is used in extract kits as the base of the beer instead of using grain.
Yeast is very important to the brewing processes because it converts the sugar into alcohol. Yeast comes either as a liquid or in dry form. Because yeast is alive, it requires the most attention of all the ingredients, needing careful attention so it does not get too warm or cold.
Hops contributes to the bitterness of the beer. It adds flavor to the brew, balancing the sweetness. Water is also very important to the brewing process, and filtered water should be used. There is a variety of hops that can be used to affect the taste of the beer, including bittering hops, flavoring hops and aroma hops. Brewers use hops to affect those qualities of the beer as desired.
Priming sugar is an optional ingredient that ensures that the carbonation is consistent among filled bottles. A muslin bag makes sure that the flavor of the beer is captured without submerging the ingredients fully into the brew. Speciality grains add depth and complexity to the flavor that is not achieved from using just the extract.