The companies involved are mainly membership organisations, and as a member you get to advertise your house and when you want to swop it, and view what other members are offering. It is then up to you to agree the terms on which you swap houses.
http://www.knowyourtrade.com/ has a full list of appropriate websites, and access to ratings, reviews and opinions
http://www.homeexchange.com/ one of the original companies in the business
www.intervac.com claims to have started the home exchange business
and for a site which is exclusively for swaps of vacation or second homes, see http://www.thevacationexchange.com They cater for non-mutual and non-simultaneous swaps.