comprehensive - covers damage to your vehicle and other property (usually to the tune of 10mil or something)
3rd party - same as above except you have limited (3-5k) or no coverage for your own vehicle
fire/theft - self explanatory
generally speaking, for most p platers/under 25s theres no advantage to comp unless you plan on writing off your car
you pay through the nose, as katie said most minor bingles will cost less than 1 years premium + excesses to fix (that's not considering increases in future premiums if you do claim)
most companies will give you a no claim bonus if you don't have any crashes anyway
having said that i have full comp on 2 cars + bike, all registered under a parents name because it costs like 1/5th as much that way
to each their own, you're an absolute idiot if you don't get at least 3rd party
if you can afford to buy another car if yours is suddenly ~not drivable~, then I wouldn't worry about comprehensive