In this blog post, Jesper Osgaard covers some of the details for CRM Online User Subscription Licenses (USLs) Enterprise, Professional, Basic, and Essential

• Dynamics CRM Online Enterprise. For your marketing and customer service departments, Dynamics CRM Online Enterprise provides licensed users with access to all of the capabilities of Dynamics CRM Online Professional plus Microsoft Dynamics Marketing Enterprise and Parature Enterprise functionality.
• Dynamics CRM Online Professional is the recommended choice your sales teams. It provides licensed users with access to sales, service, and marketing capabilities for a significantly lower price than comparable offerings from other vendors.
• Dynamics CRM Online Basic is designed for entry level CRM users who need access to basic CRM functionality such as account, contact, lead, and case management. This license is also designed for business analysts who require reporting capabilities of Dynamics CRM.
• Dynamics CRM Online Essential is designed for organizational users who are not necessarily tied to sales, services, or marketing functions but still require specific access to activities management and feeds.

Read the whole blogpost by Jesper on technet