In theory, all the systems show the same data; in practice, however, they get a little out of sync with each other. If you're looking for seats on a sold-out flight, an airline's home system is most likely to have that last, elusive seat. If you're looking for the lowest fare to somewhere, check all four systems because a fare that's marked as sold out on one system often mysteriously reappears on another system. Some airlines have rules about flight segments that are not supposed to be sold together even though they're all available, and at least once I got a cheap US Airways ticket on Expedia, which didn't know about all the US Airways rules even though I couldn't get it on their own site or Travelocity which did know about them. On the other hand, many airlines have available some special deals that are only on their own Web sites and maybe a few of the online agencies. Confused? You should be. We are.
Travel by water often provided more comfort and speed than land-travel, at least until the advent of a network of railways in the 19th century. Travel for the purpose of tourism is reported to have started around this time when people began to travel for fun as travel was no longer a hard and challenging task. This was capitalised on by people like Thomas Cook selling tourism packages where trains and hotels were booked together. Airships and airplanes took over much of the role of long-distance surface travel in the 20th century, notably after the second World War where there was a surplus of both aircraft and pilots.