Winter cycling is cool. Sometimes it is actually cold. But, with the proper clothes and equipment, it is a great way to be outside and experience the awe and beauty of winter in both the city and on the trail.
The hearty winter cyclists that keep the pedals turning all year long are brethren to skiers, ice skaters and ice fisherman, and use some of the same equipment.
Warm mittens, snow boots and pants, parkas and goggles are all non-cycling specific clothing that are perfect for winter cycling.
Then there is the bike; skinny tires are out, replaced by knobby or other specialty tires for needed traction in snow and ice, disc brakes are good for more consistent stopping power, and a hearty frame will better withstand lots of salt.