There are only four ingredients required to make beer: malt, hops, yeast and water. Though there are only four essential ingredients for brewing craft beer, each one plays a crucial role in the brewing process and the final flavour of your brew.  For example, if you add your brew hops closer to the beginning of the boiling process the flavour will retain more of the strong bitter flavour, whereas closer to the end will minimize the bitterness and favour aeormatic flavours.  You can add other flavours and ingredients like fruits or spices in order to experiment even more. A rule of thumb is about 1/2 lb or 200 gr of strong fruit flavours (raspberry,  blackberry) for every gallon of home brew, while something like peach or bananas would need twice as much to impact the flavour. Information for appentices to brewing can a guide to the process of making your brew hops into craft beer here.