Once you’ve extracted a good sugary liquid, free from grain residue, it’s time to boil it. Depending on the recipe, boiling time may be from 45 to 90 minutes. At this stage you will add the most important "seasoning" to the future beer: hops. Hops contain certain resins and alpha-acids which play important role for the bitterness, taste and aroma of the finished beer. Adding hops usually occurs in more than one steps during boiling in order to extract the desired substances optimally.
Alpha-acids are most popular among the composition of hops. Their concentration is usually printed on the label (e.g. Citra, 12.3%). They are mainly credited for the bitterness of beer. The hop that you would use for bitterness (called primary hops) is put in the beginning of the boil, so that it can spend maximum time at high temperature, which would convert the alpha-acids into something water-soluble that you can taste. Around the end of the boiling time you would add aromatic hops once or twice again, this time for flavor and aroma. These hops would stay for short amount of time at high temperature, releasing little or no bitterness, but preserving their other active ingredients.
Some recipes involve adding other ingredients around the end of the boil, like Irish moss (for better clarity) or special touches of cocoa, orange zest, lactose, vanilla, spices and herbs, coffee and other.