A piano has usually got 88 keys. When hit, they make little hammers made of wood and felt fly up and hit strings. These strings are made out of a special alloy and are then tightened. Very tightly tightened. This is why a piano's frame is made out of a special bronze alloy, too: The total pull of the strings is in the hundred kilonewton range. And don't think there's just one string per key. Some have three, some only one, some two. On average, there's more than two per key.
The mechanism that relays the keypress to the hammer is very cunning and complex. If you hit a key several times in a row very quickly, you'll notice that you can thus make the hammer bounce up and down without it or the key itself returning to a complete rest at any moment. It has taken the piano industry nearly 200 years to perfect this thing; it's called repetitive action. Experience some of it: listen to Billy Joel's Prelude.
Playing the piano seems easy. If you've never played one, it'll take you months before you get the first acceptable tone out of a violin. Yet, with a piano, even a brain-amputated clone serving as an organ depot could produce a seemingly perfect note by simply pressing a key. The problem is that piano music may require you to hit up to ten (sometimes more!) keys at once. And much piano music requires you to hit them in very intricate patterns, exactly on time; and many pieces by, say, Chopin require you to hit keys faster than a submachine gun fires. A piano is, however, not an assault weapon.
The secret for the piano's success is that it's expressive, i.e. unlike a harpsichord, it produces louder notes when you hit the keys harder, and there are pedals to control the dampers and such for more dynamic special effects. Another big selling point is that a piano is very rugged and durable. Most pianos barely need to be tuned every two years. A harpsichord must be tuned daily; a violin, every time you play it.
When playing the piano, every note should be perfect with respect to:
- its volume
- the speed, hardness and attitude of its attack (i.e. the initial hitting of the key)
- its duration
- the speed and attitude of the key's release
- the exact position of the pedal that holds the dampers up
Playing the piano is a lot like sex and martial arts combined. If you want to know what can be done with a piano, check out not only the abovementioned Prelude, but also some of Beethoven's sonatas and especially some etudes by Chopin.