Hosts specify how many days and how many guests can stay along with a short comment with other restrictions or interests (no smokers, bring a tent, young people especially welcome, etc). The hosts enjoy having people from various countries stay at their homes while the guests enjoy free housing pretty much anywhere in the world. Also, guests will feel more at ease because their hosts can help them with transportation, knowing the area, and best of all they'll be able to communicate easily through the interlanguage, Esperanto.
The official website of the Pasporta Servo: http://home.planet.nl/~lide/paspserv.htm