Camera Control

Use the mouse to change the camera's angle (your view of the world) or to zoom in and out on the action. You can also invert the camera controls (a common choice for players of first person shooters) in-game via the Menu > Options > Control Setup page.
Zoom
Scroll the mouse wheel forward and backward to zoom in and out on your character and the environment.

Rotate Camera
Hold down the right mouse button and move the mouse to change the camera angle (also known as "mouselook"). http://www.guildwars.com/gameplay/exploring/gettingaround/