The boss has just asked you to book her some cheap flights! It's every personal assistant's nightmare, being expected to perform a task at the speed of light, when you have never done it before but certainly never let on in your resume. Making travel arrangements may have come under that catch-all heading 'multi-tasking, versatile, and quick to pick up new skills.' You have probably had personal experience of booking your own vacation through a local travel agent. You know those online travel sites exist, but what do you look for to build up that online relationship you can trust? Why don't you go direct to the major airline sites? It's going to be a fast learning curve!
Google up online travel sites and the first challenge for the day will be to get off the first page.
There are just so many to sort through. You will clearly be attracted to the ones that advertise domestic fares for your region. Then it's a matter of comparing a handful of sites that seem real user-friendly to both corporate and personal clients. The best offer booking options, provide plenty of detail about the services they provide including their service fee and how to use their flight search options. Voila! you just found one that has a preferred airline search. Some allow you to search flight status, and offer travel insurance. Be sure to check out the itinerary change fees. And be sure to choose a site with telephone support. Additional features, absolute time savers for busy p.a.'s, are passport information services and driving directions with maps.
So, in answer to your question, on these sites you get a comparison including the fares offered by your favorite airline, in addition to the published fares you will get 'web only' promotions, and the advantage of bulk rates. You will still have to shop around for cheap international flights bearing in mind that rival travel agencies buying from consolidator airfares charge different mark-ups so the price for the same flight could be different on each site.