An example of proper tamping and insertion of the portafilter.
When tamping is done, the portafilter is ready to be locked into the espresso machine. There is a rubber gasket that seals the rim of the metal basket to the Grouphead, which is the spout from which water pours. At this point, the barista activates the brew cycle which forces water at a pressure of 9 bar (about 144 PSI) through the compressed puck. Since the coffee is providing resistance, it takes around 20-30 seconds for 2 ounces of water to pass through. While the water is coming through the coffee it extracts oils, sugars, CO2, and lots of other solids within the coffee.
High caliber espresso requires great attention to detail; temperature, amount of coffee, consistency of tamping, and time of extraction all play an important role in the brewing process. If any of these factors are changed, the flavor, mouthfeel, sweetness, and balance are all affected. You will undoubtedly find a huge variety of opinions about proper temperatures and all of the other variables that go into espresso extraction.
An example of 2 opinions are the traditional, Italian methods of producing espresso versus what many American coffee shops are doing. A hypothetical Italian recipe for espresso could consist of: a dark roasted blend of several coffees, exactly 14 grams measured out into a basket, a very fine espresso grind with a light tamp, a temperature of 196F, and an extraction time of around 40 seconds. An American barista may use a recipe such as: a light roasted single origin espresso, 20 grams of coffee, a coarser espresso grind, a hard tamp (40lbs or so), a temperature of 203F, and an extraction time of 28 seconds. While both of these baristas are producing espresso, the resulting brews will be very different. The Italian espresso will likely have a smoky flavor (due to darker roasting), a light body (less total coffee used), and a great deal of balance (due to blending of coffees). In contrast, the American espresso will have a citric or bright flavor (due to lighter roasting), a heavy body (greater amount of coffee used), and more emphasis on the flavors to be found within the single coffee used (single origin as opposed to blend). This variety of opinion stems from the inherent subjectivity of tasting in general. Also, despite the knowledge had by coffee professionals and enthusiasts, there is much to still learn about espresso on a scientific level. As a whole, the coffee community at large is seeking to understand and experiment more and more with this elusive brewing method.
In the future we may discuss how each parameter affects flavor components directly and other more in-depth ideas. My parting request is that you would keep an open mind when it comes to espresso and coffee in general. There is plenty of room for preference and it really is all about what tastes good… which is of course completely subjective.