Q: I read recently about ``meta'' airfare search Web sites. Can you explain how these sites are different from the airfare sites I most often use, including Expedia and Travelocity?

A: Travelocity and Expedia are online travel agencies (OTAs), which sell airline tickets and other travel products directly. But meta search sites such as TripAdvisor.com/flights, Travelzoo's fly.com, Momondo.com, dohop.com, kayak.com and Bing.com/travel search many different Web sites, including OTAs themselves and airline sites, to compare prices. One thing that meta search engines do especially well is finding airfares that the airlines sell only on their own sites. For example, Hawaiian Airlines often has special sales that can only be booked at hawaiianair.com. An online travel agency such as Travelocity, Expedia, Orbitz and Cheapair will not find these airline-site-only fares.

But OTAs have their advantages over both metas and airline Web sites, and should always be checked as well. Sometimes your cheapest flight will be outbound on United and returning on American. Delta.com is just not going to tell you this, obviously preferring to show you flights only on Delta. An OTA will, which is why many times even the metas send you only to online travel agencies rather than to airline sites directly.

And metas are not very good at flexible search. Sure, you can perhaps search one to three days in either direction of your primary dates, but that's about it. What if fares to Cancun are hundreds less in February than in March? A meta will make it much harder for you to discover this than an OTA will, since OTAs have much more robust flexible date searches. Travelocity will search over as many as 330 days at a glance, and Orbitz up to 30 days at a time.

George Hobica is founder of the low-airfare listing site Airfarewatchdog.com.