Today I just discovered a search engine website who offered a service to enter your departure destination, and then search for which possible destinations you can have for the cheapest price. This is very nice to use, if one wants to flight somewhere but doesn't know which "good deals" are available.

This is the site: http://www.kayak.com/explore/

Can someone explain to me, which programs are (mostly) used, and summarize how to make this sort of search engine. I think this is very interesting but unfortunately there are not shown all the possible flight tickets and therefore I think this project could be improved.