Next you need to condense and flavor the wort (this pre-beer liquid), which done by bringing it to a boil and adding hops according to a “hopping” schedule. The hops that are added earlier in the boil contribute a bitter flavor and acidity to the beer, while the hops that are added later or at the end of the boil contribute to the aroma of the beer. Hops add more than just flavor; they also are a natural preservative, and promote head stabilization. Gypsum and Irish moss are also added to the wort during this stage to improve the clarity of the beer and reduce suspended sediments.
The variety and type of hops you add should compliment the type of beer you’re trying to make. American hops with high alpha acids and floral or citrus flavors, such as Centennial or Galena, are used for bitter beers like IPAs. More mellow and earthy hops, like Goldings or Fuggles are used in English ales, while mellow herbal hops like Saaz compliment Pilseners.