In Microsoft Dynamics CRM buiness rule we use two types of fields the one which we are using in our “if” condition and other one which we are calculating or taking an “Action”.

if you have open the form editor and click on a field property  , by navigating to business rule tab you will see , CRM will tell you the current field is a primary field in which business rule and participating on which business rule:

so the difference is :

1. Type – Primary : this field is being used in business rule in a “if” conidition and its the base of business rule.
2. Type – Participating : this field is being calculated based on the other field (Action).

hope this helps!

Cheers!