For gas tank style heaters there are three common ways to vent the exhaust from your water heater.
Atmospheric Venting is the the most common venting system. You'll know it is atmospheric if the vent pipe goes through the ceiling and the vent pipe has a little space between the top of the water heater and the venting.
Direct Venting is commonly used to vent directly outside the space the the water heater is in and connects directly to your water heater with no space in between the top of the heater and the vent pipe. So the vent does a 90 degree turn directly outside the wall. If you have a direct vent water heater currently typically you have to replace with the same water heater. Below is an image of a direct vent model:
Power venting is common when you have to vent the exhaust a long horizontal distance. Power vents have a motor attached to assist the exhaust.