Do you have an 'S'-trap under the sink (if you waste pipe goes into the floor?) Do you have a 'P'-trap under your sink (if you waste pipe goes into the wall?) In short, its purpose is to keep sewer gasses from rising back up into the bathroom.
Here is a good picture of a P-trap configuration Full-version Bathroom Sink. Here is a cut-away version of the same showing water in the trap (and how it blocks the gases)Cut-away Bathroom Sink. It may take a little time for these 2 WEB pages to load up. If it's not happening for you, click the RELOAD button on your browser after about, say, 20 seconds or MORE.
Other than this, if you recently replaced or fixed the toilet, maybe the wax ring underneath isn't sealing properly. Or, the vent pipe for your bathroom is venting to the attic, or it's broken??? Let us know if there's more.