On a pipe, the bowl is on the opposite side from the mouthpiece. Sucking in (or pulling) on the mouthpiece draws air through the bowl. When fire is held above the bowl, the air pulls the flame into the weed to light it, and the smoke it creates passes into the body of the pipe and into the mouth.
On a bong, the bowl is usually a separate piece that is usually made of glass. The bowl has its own stem that fits into the stem that goes down into the water.
A basic, traditional bong therefore usually has 3 parts: the main body, the stem, and the bowl. When they are all fitted together and the body is filled with a small amount of water, weed is loaded into the bowl and it is ready to use.
The user sucks air in (pulls) from the mouthpiece of the bong. When a flame is held over the weed, the air pulls the flame into the material and the smoke that is produced moves from the bowl down the stem, into the water. The smoke then gathers in the body of the bong.
The user then stops pulling, lifts the bowl from the stem, and sucks in quickly, drawing the smoke from the body of the bong into the lungs.
This separate piece, called a bowl, can be purchased at smoke shops that sell bongs and smoking accessories.
Because glass bowls are prone to breaking, it is common for bong owners to own numerous bowls throughout the life of their bong.