subdomain. A portion of a domain. For example, in the domain name corp.siroe.com, corp is a subdomain of the domain siroe.com.
vanity domain. A domain name associated with an individual user—not with a specific server or hosted domain. A vanity domain is specified by using the MailAlternateAddress attribute. The vanity domain does not have an LDAP entry for the domain name. Vanity domains are useful for individuals or small organizations desiring a customized domain name, without the administration overhead of supporting their own hosted domain. Also called custom domain.
Taken from:
http://docs.sun.com/source/816-6021-10/schgl.htm