Third Party = Damage YOU do to someone else.
Fire = Hot burny stuff.
Theft = Bike is nicked by some scrote.
I have here, in my very office, a woman who is driving without insurance.
There are MANY thousands of uninsured drivers out there. If one of them hits you, you are FUCKED.
Seriously, you get NOTHING. Bike is a write off, you're physically (and possibly permanently) damaged and you won't get a penny... and they get away with it.
Fully comp packages usually have some kind of insurance against uninsured c**ts.
Lid & Leathers cover - Expensive stuff to replace if you have an off.
Free breakdown cover - When you're stuck in the middle of nowhere, in the freezing cold and pissing rain,
Uninsured loss - Anything you're transporting (SatNav, laptop, iPod, wallet, presents...) is covered. Often extends to any accessories, too.
There are 101 reasosn to go Fully Comp!!