Force.com is a cloud computing platform as a service system from Salesforce.com, that developers use to build multitenant applications hosted on their servers as a service.
According to a September, 2009 Gartner Group report, Force.com has over 1,000 customer accounts, in addition to tens of thousands that use Force.com in conjunction with Salesforce.com. The Force.com platform runs across eight data centers, with each customer fully contained in a single data center that is replicated for availability.
Several criticisms of force.com's IDE and developer friendliness have been made, including lack of support for multiple developers, speed problems with developing on the cloud, and a failure to properly separate Salesforce.com from Force.com. The platform has been described as having potential but currently only appropriate for Salesforce.com customers who want to extend Salesforce, not for independent developers who want to use Force.com as a standalone platform.
An image of Force.com - https://www.salesforce.com/platform/