The Design of the Force.com

Force.com is the preeminent on-demand application development platform in use today, supporting some 55,000+ organizations. Individual enterprises and commercial software-as-a-service (SaaS) vendors trust the platform to deliver robust, reliable, Internet-scale applications. To meet the extreme demands of its large user population, Force.com’s foundation is a metadatadriven software architecture that enables multitenant applications.
The focus of this paper is multi-tenancy, a fundamental design approach that can dramatically improve SaaS application management. This paper defines multitenancy, explains its benefits, and demonstrates why metadata-driven architectures are the premier choice for implementing multi-tenancy.
More here and a great presentation by the Chief Architect @Salesforce