In another thread, Kylie was questioning the definition of Hors Categorie and Neil stepped in to explain the "beyond category" climbs in the tour.
I thought I'd post the best explaination I've found.
You take a Citroen 2CV (See below) and try to drive it up the climb. If it will do it in 4th gear, it's a 4th category climb. If you need to put it in third, it's a category three and so on. If it won't get up the mountain at all, it's a Hors Categorie climb.
Hope that helps.