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 dates back to antiquity where wealthy Greeks and Romans would travel for leisure to their summer homes and villas in cities such as Pompeii and Baiae. While early travel tended to be slower, more dangerous, and more dominated by trade and migration, cultural and technological advances over many years have tended to mean that travel has become easier and more accessible. Mankind has come a long way in transportation since Christopher Columbus sailed to the new world from Spain in 1492, an expedition which took over 10 weeks to arrive at the final destination; to the 21st century where aircraft allow travel from Spain to the United States overnight.