When looking to book a flight at a cheap price there are a lot of options available to you. You could speak to a travel operator, book the flight direct with the airline concerned or try and find the best deals through a screenscraper, a form of flight comparison site.
These sites use the latest technology to do your shopping for you. All you need to do is enter your flight details such as the dates of travel, airports to use and any requirements, then they can search all manner of airline websites and report back their findings.
The best sites for this are usually Kayak, Travelsupermarket and Skyscanner.