A Multi-domain SSL Certificate can secure your main domain + several SAN* domain names in one Certificate.
Multi-domain Certificates are often used for Unified Communications (UC) to secure Microsoft Exchange 2007 Server, Office Communications Server 2007 or Mobile Device Manager, as well as server names, intranet and .local names.
For example, you can secure all these domains with a single SAN Certificate:
Multi-domain SSL Certificates do not require several dedicated IP addresses for your host names.
* SAN (Subject Alternative Name)