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.
Citizens from countries participating in the US Visa Waiver Program who wish to travel by air to the United States must obtain prior approval through the Electronic System for Travel Authorization (ESTA). ESTA is used by the U.S. government to screen eligible residents of Visa Waiver Program (VWP) countries before they board a plane to the U.S to determine whether or not they pose a law enforcement or security risk. VWP passengers are encouraged to apply for an ESTA when beginning to plan travel to the United States, using the convenient ESTA online form External site which may not meet accessibility guidelines. Passengers will be required to present proof of ESTA registration at time of check in and those who have not obtained prior approval via ESTA before departure will be denied boarding.
In the late 16th century it became fashionable for young European aristocrats and wealthy upper class men to travel to significant European cities as part of their education in the arts and literature. This was known as the Grand Tour, it included cities such as London, Paris, Venice, Florence and Rome. However, The French revolution brought with it the end of the Grand Tour.[7]
IMPORTANT NOTICE: Canadians travelling for tourism purposes in a European Union country must hold a passport valid for at least three (3) months beyond the date of expected return from that country (exceptions: United Kingdom and Ireland require a valid Canadian passport only). For sun destinations, the validity period must usually be at least six (6) months beyond the date of expected return. Regulations may vary for non-Canadian passport holders.
A travel warning, travel alert, or travel advisory is an official warning statement issued by a government agency to provide information about the relative safety of travelling to or visiting one or more specific foreign countries or destinations.[1] The purpose is to enable travelers to make an informed decision about a particular travel destination, and to help travellers prepare adequately for what may be encountered on their trip. In the United States, travel warnings are issued by the Department of State and are often called warden messages.[2][3]

The confusion is even worse if you want to fly internationally. Official fares to most countries are set via a treaty organization called the IATA, so most computer systems list only IATA fares for international flights. It's easy to find entirely legal ``consolidator'' tickets sold for considerably less than the official price, however, so an online or offline agent is extremely useful for getting the best price. The airlines also can have some impressive online offers on their web sites.
Other changes include: some airports have stopped curb-side baggage check, anything vaguely resembling a knife or lighter may or may not be confiscated (although lighters suddenly stopped being dangerous a year ago), you're sometimes only allowed one carry-on plus a purse, briefcase, diaper bag or the like, non-passengers aren't allowed past security, all passengers must have a document that looks like a boarding pass at most airports to get past security, you may have to put your toothpaste and shampoo in a baggie that may have to be a one quart size, some parking areas close to terminals are closed. But check-in clerks no longer ask you whether you packed your own suitcase.