What are hacker fares on kayak.com?
I was looking at the filters and couldn't find either in the FAQ or anywhere as to what hacker fares are all about.
It's not a guess, here is the official explanation:
these itineraries require purchasing two or more one-way tickets on separate airlines, so we want to make sure everyone understands that these aren’t just regular flight options.
Edit: I am very surprised this attracted this many upvotes. Try googling kayak hacker fare, it's literally the first in the result list (not counting the top highlight box, that's not the result list):