Single-domain SSL certificates provide security for a single fully qualified domain name.
For example, if you purchase a certificate for www.example.com it will not secure secure.example.com.
Wildcard SSL certificates provide security for all the same-level subdomains of a single domain.
For example, if you purchase a certificate for example.com it will secure secure.example.com, mail.example.com, etc. However, it will not secure new.mail.example.com.
A multi-domain SSL certificate provides secured communications for multiple domains and multiple host names operating on different IP addresses.