Why does my boiler vent steam?
Assuming that your boiler and mine are similar, you have two vents or pipes and the main exhaust.

Main exhaust will emit carbon dioxide and water vapour, the products of combustion.  A modern boiler exhausts gas at a lower temperature than earlier ones, having given some heat to the incoming air, so it is possible to see the water vapour condense to form a cloud.  The same effect is seen from cars on cold mornings, once the exhaust pipe is hot and the car moving, you see little or no cloud.

There should be a "condensate" pipe, this releases water that has condensed inside the unit, again, this is to give up heat that would just be lost into the air.  This is acidic water, and needs to be drained away.  My Worcester Bosch boiler lets it out 100ml at a time, because if it is released as a trickle or drip, as early boilers did, the trickle could freeze on cold days, causing the boiler to switch itself off.

The other vent is a safety device; my fitter said "if there's a fault and the water tries to boil, it squirts out of there, rather than exploding or squirting scalding hot water inside the house."  So that's good....!

Hope this helps and isn't too patronising or inaccurate.