The Class1 validation validates your domain name. (They do it by sending you a verification link to postmaster@yourdomain.com or a similar address.)
The Class2 validation validates your identity. (You have to send them a picture of your identity card).

Now, as far as encryption goes, I don't think there's a difference between the different classes.

DISCLAIMER: I haven't used any of their certificates myself. (Yet. Except their client certificate.)