Beer gets its flavour from 4 basic sources - although other flavours may be deliberately added (fruit or spices);
Malted Grains: These provide carbohydrates for the body of the beer. Different grains can be used (mostly barley but wheat & rye are sometimes used) & they can be roasted to different levels to give different colours & flavours to a beer. A highly roasted grain can taste like coffee, bitter chocolate or treacle in a beer - for example a porter or stout. Whereas a delicately roasted grain will be more subtle & let the other flavour sources come through - for example a pilsner or pale ale.
Some flavour descriptors from malt include; biscuity, sweet, dry, caramel, toffee, licorice, treacle / molasses, grainy, full, fruity, crisp, clean, soft, harsh, roast, toast, burnt, cocoa, chocloate, coffee, tannin, bitter, rich, nutty, smoke, alcohol, blackcurrent, plum, vinous, creamy, smooth.
Water: Different waters give the beer different tastes. Hard water is traditionally used for brewing stouts & soft water for pilsners. A high sulphate content can accentuate the bitterness in the beer. A high chloride content can boost the sweetness & full mouthfeel.
Some flavour descriptors from the brewing water include; harsh, soft, bitter, flinty, metalic, acid, soapy, sweet.
Yeast: Even if the yeast if filtered out of the final product, the enzymes, esthers & other by products of cell metabolism affect the flavour of the beer. Some of these flavour compounds have taste thresholds measured in parts per billion & so are very influential in in tiny amounts. Belgian beers can have stereotypically candy floss & vanilla flavour to them. German weissebiers can have banana & cloves flavours.
Some flavour descriptors from yeast include; yeasty, bready, apples, fruity, ester, banana, candy floss, cloves, pear drops, oranges, mango, vanilla.
Hops: Different hops give different flavours to beer. Some give an earthy bitterness, some a floral citous aroma & still others give pine-like resinous flavours. An American pale ale may have pine, resinous, grapefruit flavour from the Cascade hop. An English IPA may have an earthy, peppery hop bitterness.
Some flavour descriptors from hops include; bitter, hoppy, resin, grassy, fresh, earthy, spicey, peppery, citrus, floral, oranges, grapefruit.