Force.com is a Platform as a Service (PaaS) product designed to simplify the development and deployment of cloud-based applications and websites. Developers can create apps and websites through the cloud IDE (integrated development environment) and deploy them quickly to Force.com’s multi-tenant servers.
Force.com is owned by Software as a Service (SaaS) vendor Salesforce.com, who calls the product a social and mobile app development platform. Salesforce.com also owns Heroku, another cloud development platform.
See a brief introduction to Force.com:
Take a more extensive tour of the system: