Hops are what makes beer bitter.  The sceintific name for hops is is humulus lupulus, and is in the cannabis (marijuana) family, but it doesn’t contain any of the psychoactive compounds found in its semi-legal cousin. Hops are used in Beer to provide aroma, bittering, taste, and also as a preservative.  Added in the beginning of the brew for bitterness, middle for flavoring and the end for aroma.