I know the OneWorld (BA, AA, CX, QF, etc.) ones pretty well. The exact rules and the pricing vary a little depending on which country you start in, and I suggest you talk to a good travel agent because they are a bit complicated. (What follows is from my experience on business class tickets, but I assume economy is basically the same.)
Essentially there are three-, four-, five- and (I guess six-) continent tickets. (Europe, Asia, North America, South America, Africa and Australasia). The price is set only by the number of countries your itinerary touches. (Other types of RTW ticket can be on mileage limits or sectors)
Broadly speaking you have to keep going in one-direction round the world and back to where you started. A certain amount of backtracking is allowed within continents. Rerouting is allowed at any time for a fee (75USD I believe) - this is not trivial for the person doing the ticket; in my experience you should allow up to an hour at an airport ticket desk if you find yourself having to change it en route.. . . .There is a maximum total number of flight sectors (normally nine) and you can only stop over in each city once, although there are exceptions for what are deemed "gateway" cities in the continents other than the one you started from (including HKG for example if you start in Europe). Tickets are valid for one year from first departure. "Open jaws" (where you travel between two of the cities en route by some other) means are allowed.
The pricing is not as discounted as it once was - a couple of years ago a business class RTW ticket from Taipei was cheaper than even the discounted (D-class) round trip ticket HKG-LHR. This is no longer the case, but for certain itineraries they can still be very good value.
In Europe, the tickets are significantly (~25%) cheaper bought in the euro-zone than in London. More than once I have bought tickets starting in Brussels (or similarly Paris) and flown Southampton-Brussels-Heathrow with the the SOU-BRU on AirMiles in order to say about 500 quid (and once I saved over 1000 quid on a First Class ticket this way).
There are a few other quirks, but I hope that gives you some idea. On specific prices, in December a business class three-continents bought in Paris (with a routing CDG-LHR-SFO-LAX-HKG-DPS-HKG-NRT-HKG-CDG) was a little over 4500 Euros, which I reckon is a pretty good deal.
Hope that helps.
[ 07 February 2002: Message edited by: HKGpax ]</p>