I read this article on Lonely Planet about booking an around the world plane ticket. I’d never heard of such a thing, so I decided to do some research.
Basically, you are buying the ticket through an airline alliance. You might book on Delta, but you are eligible to fly on all Delta partner airlines. The tickets must be booked in advance, and you must end in the same country to begin in, but not necessarily the same city. You also don’t have to fly to each and every city. So, you might fly to London and then take the train to Paris. Then fly from Paris to Rome. Whatever the case, you must book all travel up front. Make sure you have your routes well planned out before you go.
Star Alliance has a membership of 27 airlines. The site lets you book the Round the World Fare to over 1,300 destinations. Other sites that offer the fares are Air Treks and One World and these might be cheaper than booking with the airline directly.
Before buying do your research. If you only plan on going to one or two destinations it may not be worth it. But if you plan, like I do, to do a long trip to multiple locations, an Around the World Ticket may be an excellent option for you.