AirBnb was born when two young men needed money to pay the rent and decided to offer the couch to guests who were willing to pay a small amount for it. It is in some way as a payed version of Couch Surfing, and quite fast became the number one alternative when looking for non conventional places to stay while traveling.
From the point of view of the host, the idea is quite simple: if you have an extra space (that can range from a sofa in your living room to the entire house) you can add it to the list of AirBnb with photos, description and price. Once you did this when people starts looking for a place in your city they will be able to contact you and after reviewing the profile you can decide if he is someone you would like to have at your place or not.
From the perspective of the traveler it is also very simple: if you are tired of staying in hotels and you prefer either to have contact with locals or to have your own space with kitchen and bathroom, you can enter to AirBnb, pick the city you are visiting, the dates and the price range. Then you’ll see a listing of offers. In the case of sharing the space (a room in a house, for instance), it is important to check the profile of your host and decide if you want to stay there or not.
But that is not all: if you register following this link, you will receive 22€ (25U$) for your first trip. It is almost a free night anywhere in the world, without efforts, just registering and trying it out.