This article is from the Secure Sockets Layer Discussion List FAQ, by Shannon Appel SAppel@consensus.com with numerous contributions by others.
Attribute Certificates are a new type of certificate proposed by
Netscape. These are signed objects that assert additional properties
about a particular identity certificate.
An attribute cert has no associated key pair and consequently cannot
be used to establish identity. Informally, one can think of them as
a mechanism for extending the attributes of an identity certificate
without requiring that the identity certificate be reissued.
More details of the proposal are at