Different varieties of hops have different bitterness and aroma characteristics.
For home brewers and small breweries there are equations that can be used that give an approximation of the bitterness transferred into the beer during the boil stage of brewing. Basically, the more hops added, the higher the alpha acid content of the hop and the longer its boiled for, the more bitter the beer. The units of bitterness are in EBUs (European Bitterness Units) or IBUs (International Bitterness Units).
Originally bitterness was measured by a taste panel who compared different beers with known amounts of hops in and compared them to the beer in question. This was obviously very subjective and a better method was needed.
One way to measure this is by solvent extraction of the bittering substances from beer into isooctane. The bittering substances dissolve better into organic solvents than aqueous ones. The solvent then has ultraviolet light shone through it and the amount absorbed is proportional to the bittering compounds present. Several approximations are used in this method, but is is fairly low tech, quick and the kit to use this method is only a moderate amount of money. This can still differ from the actual value by as much as 30%.
The most accurate way is to use HPLC - High Performance Liquid Chromatography. HPLC works by having a very long coiled tube with a packing material providing obstructions to the flow of substances through it. Different substances are hindered by the packing material by differing amounts as they pass through the tube. This separates out the individual substances as they move along the tube and can be detected individually as they come out of the end.
In the case of beer, the bitterness substances are extracted using methanol and injected into the apparatus. Methanol continually flows through the HPLC tube and the injected sample begins to separate out as it moves through the tube into its individual components. A spectrophotometer measures the change in absorbance of different wavelengths of light as the individual substances pass through the end of the tube producing a series of spikes on a graph. From information found by running standards through the HPLC, an individual spike can be identified as a compound and the amount calculated by the area under the spike. Breweries use this to standardise their beers between batches and also check that certain compounds that cause light strike are not present before bottling the beer.
The perception of bitterness on the human palate though varies with just about every parameter in beer. Even though a beer may have 30 IBUs of bitterness, the taste of bitterness will vary with each hop variety, alcoholic strength of beer and the gravity of the beer.