A Multi-domain SSL Certificate can secure your main domain + several SAN (Subject Alternative Name) domain names in one Certificate.
Multi-domain Certificates are often used for Unified Communications (UC) to secure Microsoft Exchange 2010 Server, Office Communications Server 2007 or Mobile Device Manager.
For example, you can secure all these domains with a single SAN Certificate:
www.namecheap.com
www.exchange.namecheap.com
www.namecheapgroup.com
www.web-hosting.com
Multi-domain SSL Certificates do not require several dedicated IP addresses for your host names.