Salesforce.com is SaaS (Software-as-a-Service) product while Force.com is a PaaS product (Platform-as-a-Service). Salesforce.com has a selection of prepackaged solutions such as the Sales & Service Cloud that are designed for a specific purpose. While Force.com allows you to build your own applications. Salesforce.com is built on the Force.com platform.
Force.com is the platform on which developers can build custom Salesforce application and thus a service offering of Salesforce. The exact difference between Salesforce.com and Force.com isexplained in the below paragraph. Salesforce is the company (Salesforce.com) which provides cloud computing services.