An instance of the Force.com platform that has active paying users accessing business critical data

Production Force.com environments store the live data that is actively used to run your business. Most of the production environments in use today are Salesforce CRM customers who purchased Group Edition, Professional Edition, Enterprise Edition, or Unlimited Edition. If you do not require any of the Salesforce CRM functionality, you can sign up for Force.com Edition. When developing an application to use internally, you typically know the features and limits of your production environment. However, as a partner looking to build a Force.com application with commercial intent, it's important to understand the different environments, thereby ensuring that the application you build will work in the desired environment. You can find a detailed feature comparison guide of the production Sales Cloud, Service Cloud and Force.com Edition environments on their respective pages. You can develop a custom Force.com application for your own production use, for existing salesforce.com customers, or for any business or organization in the world. When building an application, it is recommended you use a development environment.

An Introduction to Environments