Drain gurgles when toilet is flushed or sink drains?

very simple, your drain that is gurgling is not vented properly. when the toilet is flushed, it is a large amount of water going down the stack at once. that sudden void creates a vacuum. it will pull air from the closest points possible. in this case, your gurgling sing or tub. if the tub and sink were vented, the air would be pulled from the vent stack, rather than the trap. hope this helps.

I think this answer is incomplete. The drain can also gurgle due to a blockage down the line. Check to see if water is comming up anywhere when you run lots of water, if so call a plumber to snake the drain.

99% of the time, it's related to venting. Most drain blockages will cause drains to empty slowly but not gurgling. The source of the vent problem can vary, but the most common are that the vent was never installed, or the vent is plugged from something that entered from the top (bird, animal, ash, leaves, dirt, snow, etc.), or the vent is plugged by something that entered from the bottom (leftover material from a backed-up septic/sewer line). All are pretty easy and cheap to fix yourself but it can be a little messy (and smelly). Keep an old towel handy and wash your hands when done ;-)