Eclipse is a proc that happens when you crit with either wrath of starfire when you have the talent. When you crit with one and it procs eclipse, it increases the damage of the other spell. The proc for each kind of eclipse is on a 15 sec cooldown, so you can't proc the damage increase to starfire more than once every fifteen seconds, but you can proc the damage increase for wrath while the starfire eclipse is on cooldown.
In short, with Eclipse, your DPS rotation looks like this....
Wrath til crit causes Eclipse-> Starfire til crit procs Eclipse -> Repeat...Innervate, Wrath of Nature and Trinket as necessary (or whenever they are off Cooldown), toss in Hurricane or Cyclone when necessary. Your DPS cooldowns are normally used at the start of a boss fight so that you have a possibility of them coming off cooldown later in the fight so you can use them again, which therefore will increase your total damage done for the fight.