Salesforce.com is Sales(CRM) and Service applications which were developed and running on the Force.com Platform. The developed applications will run on the force.com always as a multitenent architecture method. Simply you can identify Force.com is where you can access and customize your datamodel, bussiness logic and UserInterface. But in case of salesforce.com you can access the sales(CRM) Application with all the developed data model, Business logic and UserInterface. Since it is running on the force.com platform, you can customize your application funtionality.

     Force.com is based on the concept of Platform as a Service or commonly known as PAAS where as Salesforce.com is based on the concept of Application as a Service. So, we can say Salesforce.com is an application built by Salesforce on force.com platform and has been made available to people for use at per month per license pricing.