no water in toilet bowl
As Carl noted above, if a pipe/system cannot pull fresh air in as water fills up the pipe on it's way out, a siphoning affect is created which can pull the water out of the 'u bend' in the toilet during a flush. 

The vent is usually the last or highest part in the system. Look for the pipe outside as it vents into fresh air, usually through the roof or on an outside wall (in the UK at least).

It's a bit rare for a vent to get blocked though (birds and animals occasionally) as the pipe normally only has air in it! It's possible the pipe is blocked where the bottom of the vent pipe joins the sewer pipe (if the pipe gets backed up or such like).