here you can find a complete comparison table for the high difficulties.
I've limited my chart for some simple reasons :
beyond the IV UIAA only skilled climbers can go, and skilled climbers do not need explanations, they should have tried :)
On the contrary I've seen many not skilled (or beginners) climbers thinking that they're ready for a III UIAA climb after having climbed simple real I UIAA routes, so that they're going to meet some problems and some risks ...
Gabriele, could you also add some pointers to equivalent grades in other systems, like the YDS ?
I used to think UIAA I was some kind of Class 3, UIAA II a Class 4 and UIAA 3, 3+ some kind of Class 5 to 5.4, yet I am not sure of the accuracy here...