Though bongs themselves come in many shapes and sizes, and also with many features and quirks, there is a basic principle.

Simply put; a 'bong' is a smoking instrument that cools/filters smoke by allowing it to pass through a fluid, typically water, before being inhaled. If a substance can be smoked, it can be smoked in a bong. The smoker may have to adapt the 'mix' to get the most out of the substance they smoke in the bong however.

As you can see in the diagram, the 'mix' is placed in a 'cone' which sits on the end of a pipe. Call it a 'down-pipe'.
The other end of the down-pipe is submerged in an almost sealed container, half filled with a fluid, typically water.
A hole is made near the top of the container; this is the rush-hole.
Another pipe extends from the upper half of the container. Call it the 'mouth-piece'.
The mouth-piece is used to inhale the smoke that has travelled down the down-pipe, through the fluid in the base of the container and finally, up, towards the mouth piece, your salivating lips, the deepest recesses of your lungs and the furthest borders of you Mind, maaaaaan!
Please note that bongs only grow chicken legs, after an EXTREMELY heavy session!

Home-made bongs are fun to make, a ritual if you like. But, more often than not, the materials used in their construction do not stand up to the test of time and the rigours of even moderate use. Quality glass bongs, hardy acrylics, and durable metals are all professionally engineered by companies, run by people who are empathetic to smokers and their life-styles and tastes.

Making a bong with materials found around the home is certainly possible, but you should be aware of the properties of the materials you are using. Plastic pipes are BAD. They often release toxic fumes when heated and burnt. Some people use aluminium foil as a gauze or 'cone'. Aluminium releases fumes when heated and burnt. These fumes are many times worse for your lungs and health than tobacco or any other herbal substances.

Plenty of people take pride in their own home-made bong. An afternoon in a hardware store can be something of a comically religious experience for the person shopping for bits and pieces with which to build their own bong. But it can be a chore when it comes to planning its construction, especially when you might want extra chambers for more water, hot water, maybe just ice, to cool and filter the smoke.