The thing that makes espresso unique amongst all coffee brewing methods is its use of CO2 in the extraction process. Under the conditions of temperature and pressure encountered in pulling a shot, the CO2 from the coffee dissolves in the water forming carbonic acid which lowers the pH (acidity) of the water. This in turn affects the relative solubility of the hundreds of components that reside in the coffee grounds and it changes the profile of the compounds that are extracted. As the extract starts to pour from the basket and it returns to ambient pressure, the dissolved CO2 boils off and creates that honey-like crema which lets us know that we are in the sweet spot. Not too different from popping the cork on a fine bottle of champagne!
Bob Barraza wrote much more on the subject… very enlightening on the subject of getting the biggest bang out of your beans… and pocketbook.