Couchsurfing, much like Hospitality Club, is a large active online community which facilitates cheap travel and meeting locals. It has members from all over the world, who offer up their couch as somewhere for travellers to crash on for a few nights (for free of course). It is a two way system, so if you join, you must also offer up your couch to travellers who want to visit your part of the world.
Everyone has a profile explaining the rules of their house, where their guest would sleep (they usually don't really have to sleep on a couch if the host has a guest bedroom), how many nights they generally let people stay, how many people they accept at a time, and most importantly photos and personal interests.
There is a comment and rating system, so after someone visits a house they usually leave a little note that shows up on their host's profile where everyone can see it. This is important because it is the only safety system. You can read people's comments to see if past travellers have had good or bad experiences with them, and people who had bad experiences really are pressured to write them up as a warning for others.
To contact people on couchsurfing to ask to surf their couch you must join, and use an email system inside the website to leave them a message and ask if you can stay on such and such dates (no one is obligated to say yes of course).
This is the best way to travel for me. I joined (like most people) because I didn't want to spend money on hostals, but the best part of couchsurfing is of course the people you end up meeting. You have someone to ask about the city (and some people even show you around their city), and many times people gave me food, took me out, kept me company and made me feel safe and welcome in a foreign city.