What are 'rotations' and how important are they?

[Tamo 49]

What are 'rotations', and how important are they? Furthermore, where can I find a good rotation for a particular build?

[Arvinthir 38]

Rotations are basically the order in which you prioritize using your spells so as to maximize damage dealt. They are fairly important, especially on bosses that are heavy dps checks (like Gorseval). 

qtfy.eu has written and video rotations for all meta classes.