In the context of air travel, what is the same vs different between the terms "transit" and "transfer"?

There are multiple uses of transfer and transit even within the field of travel, so context is essential. Much as many travelers indiscriminately say direct when they really mean nonstop, precision in your own terminology and tolerance for others' will be necessary for understanding. Some of the key differences lie between

• Industry use and general use
• American English and British English.

To a travel agent, a transfer outside of other context is transportation on the ground between two locations. The National Tour Association (NTA), a US-based travel and tour industry group, defines transfer in its glossary as

Local transportation and porterage from one carrier terminal to another, from a terminal to a hotel, or from a hotel to an attraction.

In other words, if you need to get from JFK to LaGuardia, from LaGuardia to the Plaza Hotel, or from the Plaza Hotel to the Lincoln Center, you're in need of a transfer. For instance, VisitLondon.com describes where to find hotel transfers at Heathrow, Viator has listings of Airport and Ground Transfers, and Virgin Atlantic hawks a chauffeur-driven ground transfer option.

This usage is not common in American English among the general public, however. Signs for airport transfer or ground transfer will be rare at a North American airport. You will invariably be directed to to ground transportation (sometimes in Canada and the Caribbean, ground transport). A transfer is the act of changing from one aircraft to another, and any signs for transfers are thus likely to refer to connecting flights.

Transit, similarly, means something different on different sides of the pond. The Oxford Dictionaries Online gives, under the first definition of transit, this sense:

1.1 North American The conveyance of passengers on public transport.

In other words, say transit out of context to an American or Canadian, and she is likely to think of mass transit or public transit— buses, Metro and subway lines, light rail; transit is in the name of many operators. (I unsuccessfully proposed to rename the transit tag for this reason.) Most people will understand that in transit means you are on your way to something, but not necessarily grasp that you have a layover at a connecting airport; a few people may misunderstand you as saying you are on transit (i.e. taking the bus or train).

The industry use of transit on the other hand, refers to stopping points before the journey has been completed. Thus, a passenger going from Chicago to Stuttgart via Zurich will be in transit at Zurich, and could be called a transit passenger. In contrast, a passenger traveling from Chicago to Zurich is ending the journey at ZRH, and thus is a terminal passenger (or origin-and-destination passenger),

But while this meaning of transit is found in related terms like transit visa and transit hotel globally, a member of the Canadian or American public would probably just describe transit passengers as transferring passengers, layover passengers or connecting passengers, even if they aren't changing aircraft.

A "transfer" is essentially moving from one item to another item. So it would apply equally to going from a hotel to an airport, as well as going from one airplane to another airplane.

The term "transit" applies more to staying temporarily before continuing on to your next destination. You would "transit" an airport if you fly from A, stop for a while, then continue another flight to B. You "transit" a subway station when you stop to change from train C to train D. And yes "transit" would apply even if you dashed from conveyance 1 to conveyance 2 in less than 30 seconds ;-)