Types of Visas to Turkey, and Legalities
Generally there are two types of tourist visas - a single-entry visa (called Tek giris) and a multiple-entry visa (called Muteaddit giris). A single-entry visa allows a one-time entry into Turkey and once you leave the country (irrespective of your length of stay) you need to obtain a separate visa to re-enter Turkey. A multiple-entry visa, on the other hand, permits multiple entries into the country within the span of its validity without incurring any additional cost. If you plan to enter and leave Turkey 4-5 times in a span of three months, for instance, it is worthwhile going for a "Muteaddit giris". Certain nationalities will automatically be granted a multiple-entry visa on payment of their fee at their port of entry.
Important note: If you plan to make use of this and leave and re-enter the country within your period of 90 days, make sure you do not overstay your welcome! The 90 days must cover you for the ENTIRE time you are in Turkey, and not just parts of that time. The penalty for over-staying your visa can be a fine (which increases depending on the time spent over the time limit), potential deportation and in some cases, a ban on returning to the country. This ban can be anywhere between three months and five years.
The visa application form entails two important sections - occupation and purpose of visit. A general tourist visa does not permit working (paid or unpaid), residing or studying in Turkey. For such purposes, you need a work/study visa as well as a residence permit.
If you are an ordinary passport holder, you cannot indulge in any diplomatic, military or official business in Turkey. For such purposes, your visa needs to be stamped with the words "official passport".
Similarly, if you want to shoot documentary films, go for an archaeological excavation or conduct research, you have to apply for prior special permission from the Turkish authority.
Visa requirements for truck drivers are also separate.