What are the different versions of Salesforce and what does each one offer?

Have a look at the link below for the different features available with different versions of Salesforce:

http://www.salesforce.com/crm/editions-pricing.jsp

Vertical Response is designed to work with Group, Professional, Enterprise, and Unlimited Editions. Since Group does not have access to the campaign tool, there is a slightly different build for our users with Salesforce Group Edition.