Walk
Walking is the most common movement type. All characters have a base walk speed, typically 6 squares (30 feet). Walking is affected by difficult terrain, which halves your movement through affected squares. Walking provokes opportunity attacks when you leave a threatened square.
Walking allows you to move along any solid, stable surface — floors, roads, bridges, etc. You cannot walk on liquids, through solid objects, or through the air. If you fall while walking, you take falling damage as normal.
Certain effects can increase or decrease your walk speed. A walk speed of 0 means you cannot move on foot, but you may still be able to use other movement types.