The new regulation replaces the old 90 days visa which could be purchased nonstop or renewed before expire date. Earlier you could also for example cross the border of Turkey by going to Greece in order to buy a new 90 days visa to prolong your stay in Turkey. There was also the possibility to extend your visa in Alanya so it would not be necessary to leave the country, but this possibility could only be used once.
What will be changed?
With the new regulation it is still possible to buy a visa for 90 days, but this visa will now last for a limited period of 180 days. This is also called the 90/180 rule. Multiple visits are allowed but a new visa cannot be purchased until at least one day after the current visa expires.
Did you receive a visa for less than 90 days; you could prolong your stay to the maximum of 90 days.
What does this mean in practice?
For example, on the 1st of July you are going on vacation to Turkey and you buy a visa from the airport. This visa is valid for 180 days (that is about 6 months). In this period of time it is permitted to stay 90 days in Turkey. When you leave Turkey on the 30th of July, there remains 60 days which you can use in the 180 days period of time (that is till the 1st January). When the 180 days expire your visa will not be valid anymore. You could then buy a new visa on your next visit to Turkey.
It is advisable to request a residence permit, if you stay more than 90 days (in those 180 days period) in Turkey. In that situation you have no restrictions to travel to and out Turkey. For more information about the application procedure and the prices of the residence permit see for example the website of the Foreigners Department of the Police of Antalya.
