In my opinion, the best way is to just take a cab. You can get from Manhattan to LaGuardia (or JFK) and back by subway or bus (see the Related Link below), but as far as I know, there is no direct...
Yes it is.
Not all airports are international airports, some are domestic airports.
An international airport is an airport that has flights to and from other countries and is equipped with customs and immigration facilities.
No, not all are international.