An airport transit visa may be required when you have connecting flights in your itinerary.
This is the visa you need to transfer through a country. Even if you’re not planning to leave the airport, you may still need a visa. Some reasons include:
- Rechecking your baggage
- Moving between terminals
- Multiple PNRs in your booking
If you need a visa to enter a country, then you should assume (until proven otherwise) that you will need a visa to transit through that country.
As the passenger, it is your responsibility to ensure you have the correct visas and travel documents for your trip. Contact the embassy or your foreign ministry for detailed visa information.
Wikipedia is a source of quick information for visa requirements. For example, here you can see the visa requirements for Indian citizens
Check out this FAQ as well: Do I need a visa for my flights?