Transit Visa is the type of Visa provided to step out from the airport or the seaport into that country through which you are transiting to your destination country. If we consider the most popular west destination from India, there are two routes to be considered for the travel by air or sea from India. It is the pacific route and the Atlantic route. In the pacific route, there may be stops in Frankfurt, Brussels, London, Amsterdam etc. In the Atlantic route, there may be stops in Singapore, Hongkong, Taiwan etc.
During your travel you may wish to step out of the airport during the transit time and take a tour in that country. This situation requires a transit visa to be obtained pre planned for such short tour. Some countries require a minimum of 24 hrs of transit time to provide a transit visa. But Singapore allows you to take a free transit visa even for 4 hours of transit time. Also the government provides a free tour for the transit passengers in its own vehicle to tour Singapore.
Most of the transit visas are to be acquired in the airport presenting the required documents to the authority during the transit. We cannot simply step out of any airport to tour the country. The required papers need to be presented to the airport authorities. Each country has its own requirements to provide transit visa. Please visit the website of the particular country through which you are planning to transit to know the papers to be presented and fees to be paid for the transit visas.
Most of the countries require valid flight ticket and valid passport of native country and visa for your final destination country to take a transit in its airport. Just utilizing the airport during your transit time doesn’t require any transit visa. United Kingdom requires the transit passengers who step out of its London Heathrow airport and catch their next flight from London Gatwick need to acquire their transit visa in the Heathrow airport as we are stepping into the country land.
Remember, to step out of the airport to tour the country through which you are transiting might only require a transit visa. You need not worry of transit visa when your flight itenary has transit in multiple countries before your final destination.