Basically there are two main categories of Company Registration Jurisdictions in UAE in which both are 100% tax-free:

1- Mainland or Economic Development Department:

 Department of Economic Development (or Economic Department) is a governmental body responsible for issuing Trade Licenses (Company Registrations). Each of the 7 emirates in UAE has their own Economic Department acting almost independently under the Federal Rules of UAE. What you need to know about business setup under Economic Department Jurisdiction are:

a. The company shares has to be divided 51% local ownership (UAE Citizens) and 49% foreign ownership
b. The company is allowed to conduct trades directly inside UAE Market

2- Free Trade Zones are special zones inside UAE with the goal of promoting foreign trade in UAE. Each Free Trade Zone is being managed by an individual governmental authority. All Free Trade Zones are operating under the set of rules from Federal Government but with great level of independency.
There are more than 27 Free Trade Zones in UAE in different emirates. Some of these zones are more of general free zones issuing different types of licenses and activities. Some are specialized zones registering only companies with special activities like Dubai Media City which is only issuing trade licenses for the Media companies.
What you need to know about business setup under Free Zone Jurisdiction are:

a. The company share ownership can be 100% foreign ownership
b. The company is not allowed to conduct direct business inside UAE. But in most cases it can do business through a Local Agent. Local Agent is a company registered under the Economic Department Jurisdiction.
c. Unless the imported goods are not released from the Free Zone there will be no import tax imposed on the goods. But it is also possible to enter the goods inside UAE market by paying the import tax of 5%.