First class flights cost up to ten times economy fares. Advantages include express check-in, special airport lounges, very comfortable seating with loads of legroom, luxury food, etc. (Plus, of course, the chance to look down your nose at economy class passengers).
Example, London-Sydney, out on 5th December back on the 12th, with Quantas:
Economy fares vary between �729.20 and �788.20 depending upon which flights are selected.
First class fare (all flights) is �7049.20.
It's mostly for the super-rich, and for travel on corporate budgets. Ordinary business (Club) class is a lot cheaper (probably �3000-�4000 in the example Buenchico gave) and gives you plenty of extra comfort. A couple of business-class-only airlines - Eos and Maxjet - have started flying to NY, offering business class a lot cheaper still than the big airlines. Maxjet introductory offer is �854 return, which is 2-3 times economy fare - not much at all.