Science: How a Toilet works.

• Pushing on the handle pulls the chain, which releases the flush valve.
• About 2 gallons (7.6 L) of water rush from the tank into the bowl in about three seconds. The flush valve then reseats.
• This rush of water activates the siphon in the bowl. The siphon sucks everything in the bowl down the drain.
• Meanwhile, when the level of the water in the tank falls, so does the float. The falling float turns on the refill valve.
• Water flowing through the refill valve refills the tank as well as the bowl. As the tank refills, the float rises, and when it reaches a certain level the refill valve shuts off.
• Should something go wrong and cause the refill valve to keep running, the overflow tube prevents a flood.

Hear Is a simple animation that sums it up well.

Sources: How Stuff Works
Image: Wikipedia