Basically, they're bulk tickets sold to travel agencies, with savings passed on to the consumer. Some airlines, like Icelandair, Citybird, Virgin Atlantic, LTU, and Martinair can almost always be counted on to have the lowest fares, or at least as low as the big-airlines' sale fares. There are exceptions, especially in summer, when the low-cost airlines work the law of supply and demand just like everyone else, and their fares are sometimes higher. You won't know for sure unless you check with them yourself. Routes, contact numbers and web site addresses for these airlines are here.
Lowest fares means lowest amount of travelers.
Sometimes not much less than the best price you could come up with otherwise. No guarantees flight you want will be available. More info here . Once you register with Airhitch they will try and get you a flight on a space available basis from a list of US and Canadian airports close to you. You have to be flexible about which airport you fly from, where you end up and the day you fly.