In the first place and just in case that you would be wondering, a transit visa is the visa needed for a person who is stopping over in a country for no more than 24 hours at a midpoint between the departure and destination points. This midpoint simply represents a "transition" point for the person travelling who will only use this point as a mean to reach his or her final destination.
Transit visa is valid for up to three months and allows the person to travel to another country through transiting the Turkish territory.
TYPES OF TRANSIT VISA
By Air
As long as you will be staying inside the "transit lounge", in other words, unless you pass through the passport control point at Turkish airports, no transit visa will be required. Please keep in mind that in those conditions you will not be allowed to leave the airport transit area. You must also already hold a complementary flight ticket to a destination outside of Turkey as well as, if needed, the permission/visa to enter within the destination country.
By Sea or Land
In the case of travel by sea or land via Turkey, a visa will be needed. The process to apply for this visa is the same than the procedure for foreigners applying for a tourist visa.
Please, note that if you are travelling to Turkey by cruises, you will then be allowed to enter the country without visa requirements for a maximum of 72 hours, with the permission from the local security authorities.
Additional information
Before applying for a transit visa, please make sure to gather all the documents required in order to obtain the visa of the final destination country of your journey since this visa is a necessary to acquire your transit visa. In the situation for which no visa is required from you to the destination country, you will still be asked to provide an official letter issued by the relevant countries' diplomatic representatives testifying that you do not need any visa to enter their country.