For most coffee drinkers, one of the key features of espresso is not just the strength of the cup but also the layer of dense foam that tops the drink. Crema is simply Italian for cream.
The reason this happens is that when water is under very high pressure it is able to dissolve more carbon dioxide, the gas present in coffee that was produced during the roasting process. When the brewed liquid gets back to normal atmospheric pressure on its way to the cup, the liquid can no longer hold on to all of the gas so it comes out of solution as innumerable tiny bubbles. These bubbles become trapped in the coffee liquid and appear as a stable foam.
For a long time crema was considered important, but in fact it can only tell you two things. Firstly, whether the coffee is relatively fresh, the longer ago it was roasted, the less carbon dioxide it will contain so the less foam it will produce. And secondly, whether the cup of espresso is strong or weak. The darker in colour the foam, the stronger the liquid will be. This is because crema is just a foam of the liquid, lighter in colour because of the way the bubbles refract light, so the colour of the coffee determines the colour of the crema. For this reason, coffee that has been roasted darker will also produce a darker crema. The crema cannot tell you if the raw coffee is good, or has been well roasted, or if the equipment used to make the espresso is clean, all key factors in a delicious cup of coffee.