The easiest option is to buy all Professional licenses because they represent the superset of all the Dynamics CRM features. Your admin users, power users, and others who use all or nearly all features will need these licenses.
At the other extreme is the Essential license. This license is for internal users (in your organization) who will interact with Dynamics CRM through custom applications or a portal, and who do not need to access the Dynamics CRM 2013 user interface at all. This is the least expensive license and might be appropriate for a large number of users with light functionality needs. For instance, if you are using Dynamics CRM for a helpdesk, you could let users start or check on the status of a trouble ticket on a web page. The hidden cost of Essential licenses is that someone must build a webpage or other application to provide a user interface to Dynamics CRM.
In the middle of these options is the Basic license. Unlike Essential, it gives the user access to the Dynamics CRM user interface, but restricts which entities are included. Be sure to take a close look at your requirements before choosing Basic instead of Professional.
For more information, contact your Microsoft representative or see the Dynamics CRM cost calculator. Complete licensing guides are available for on premises and CRM Online.