On a typical 27-speed mountain bike, about six of the gear ratios are so close to one another than you would never notice a difference between them. So why all the hoopla over more speeds?
In actual use, riders tend to choose a front sprocket suitable to the slope they're riding on and stick with it. The front sprocket is difficult to shift under load. It is much easier to shift between the gears on the rear. If riders are cranking up a hill, they'll probably chose the smallest sprocket on the front and shift between the nine gears that are available on the rear (since they can do this without easing off on the pedals). Having more speeds on the back sprocket can be advantageous.
The shifter design used on mountain bikes today recognizes that the point of shifting is to maintain a constant cadence. This means that most of the time you'll just be making small adjustments to the gear in order to increase or decrease your cadence. The front and rear shifters are equipped with switches that shift gears one at a time, either higher or lower.
Each shift lever adjusts a cable that determines the position of the derailleur. Both the front and rear derailleurs contain strong springs that force them to one side or the other. The shift lever either pulls against those springs to move the derailleur one way, or lets the springs pull the cable to move it the other way.
Now let's take a look at some mountain-bike suspension systems.