Airport transit visa = Airside transit, you will not enter the country, no passport control, you can't leave the airport. It's required for some nationalities. You won't need it if you are:
- If the nationals of these countries hold a valid/unexpired USA Visa;
- If the traveler is returning from a legit stay in the USA and the trip takes at least 4 days prior to the visa expiration date.
- If the traveler holds a valid residency permit allowing him to move in and out of the USA.
- If the traveler holds a residency permit that allows unrestricted return from Andorra, Japan, Monaco, Canada or San Marino.
- Holders of a valid uniform visa, national long-stay visa or residence permit issued by a Schengen state.
Transit = landside transit, you will enter the country by passing the passport control, you can leave the airport to catch your flight in another Schengen area airport. This is basically a Schengen type C short visit visa. Unless of course you do not originally need a visa to visit the Schengen area.