Pretty much what the question says. What's the difference between the two classes of roots? The differences between the certificates signed by such roots? What uses would a class 1 signed certificate have that a class 3 doesn't, and vice versa?
The class 3 root certificate is the high-security subset of the CAcert class 1 root certificate.
Class 1 is the 'normal' and older root certificate of CAcert. It includes both, low security and high security certificates. As it might not be possible to get the class 1 certificate included into some browsers or distributions, the Class 3 certificate was introduced. The Class 3 root certificate includes only high security certificates and is a subset of the Class 1 certificate.
In general: The class 3 will probably be integrated into more browsers and distributions in the future, whereas the class 1 certificate probably works with more and especially older browsers.
(See http://www.luga.at/mailing-lists/luga/2006/02/msg00109.html)