Force.com is a platform where you can develop your own application.
Salesforce.com is Sales(CRM) and Service applications which were developed and running on the Force.com Platform.
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.
The developed applications will run on the force.com always as a multitenent architecture method.
Simply you can identify Forc.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. Then you can customize your application funtionality, since it is running on the force.com platform.
So, we can say Salesforce.com is an application build by SFDC on force.com platform and has been made available to people for use at a per month per license pricing.
As a rule of thumb: Force.com platform license is three times cheaper then the Salesforce.com license but takes three times of effort and time to build any functionality which is billed in Salesforce.com application.
Force.com is the platform because of which SFDC says sky is the limit for developing any application.