Walls, doors, large pillars, and various obstacles that fill squares on the battle grid are blocking terrain, which prevents movement.
Blocks Movement: Creatures can’t enter squares of blocking terrain. A typical square of blocking terrain is completely filled, which prevents diagonal movement across its corners.
No Line of Effect or Line of Sight: Blocking terrain blocks line of effect. It blocks line of sight as well, unless the terrain is transparent.
Cover: A creature can gain cover by positioning itself near blocking terrain. See “Cover and Concealment".
Published in Player's Handbook 3, page(s) 222, Rules Compendium, page(s) 309, Monster Vault: Threats to the Nentir Vale, page(s) 122.