To figure out what might be causing that awful smell, it helps to have a basic understanding of how the plumbing in your bathroom works. (Don't worry -- this will be quick!)
At some point, you've probably looked under the bathroom sink and noticed the U-shaped pipe that runs from your sink drain (B) to a larger wastewater pipe in the wall. One end of that big wastewater pipe runs down to your sewer or septic system (E); the other end leads all the way up through the roof (D), letting fresh air in while allowing any smelly sewer gases to vent out to the sky. And although you can't see it, the same basic drain setup is at work behind or beneath your tub and shower (C).
The U-shaped section of pipe below your sink is called a P-trap. When everything's working as it should, a small amount of water stays in the P-trap after you turn off the sink, empty the tub or flush the toilet. That little bit of water is enough to prevent gases from drifting out of the sewage system and into your house. Under normal conditions, those gases flow right past your bathroom and out through the vent pipe in your roof.
Sounds simple enough, right? But if you have a sewage smell in your bathroom, it usually means something in that very basic system has gone wrong. If you're lucky, it will be one of the easy fixes on the next page.