Difference between force.com and salesforce.com?

Different licences. Salesforce.com licences are for the Sales Cloud CRM product. Force.com is the platform everything is built on, and you can buy Force.com licences to build a custom product that does not utilise the standard CRM features and objects, such as Opportunities for instance.

IMHO this is the best aswer: https://success.salesforce.com/questionDetail?qid=a1X30000000btOsEAI

There are at least three different things going on in your post, from what I can tell

First of all, Salesforce.com is built on the Force.com platform - they are more or less interchangeable from a technical standpoint. It's a mere matter of marketing semantics. Force.com applies to the infrastructure and codebase that is the foundation for the entire Salesforce solution. They are enforcing the Force.com concept more so because Salesforce can do so much more than CRM nowadays.

Secondly, regarding the functionality situation. In this case I think you are referring to the Force.com Edition in comparison to the standard Salesforce.com Editions (Professional, Enterprise, etc.). In this case, yes there is a difference. The Force.com Edition (or Platform Edition, depending on the marketing materials you have in hand, what day of the week it is and whether or not your sneakers have green stripes) is a standard Enterprise Edition (if you buy in conjunction with a full Enterprise Edition) minus the CRM functionality - so no Leads, Opportunities, etc. (as far as I know). This Platform Edition is aimed at providing customers with the "platform" on which to build custom applications - for companies that do not so much use Salesforce for the CRM.

And lastly, regarding the Developer Edition accounts you created with interests in Salesforce.com customization and Force.com platform - again, just another marketing ploy. Salesforce uses the preferences you set to send you more targeted information via email regarding the topics of interest to you.