How is a trailer attached to a tractor?

Depends on what type of tractor and trailer you had in mind. For a farm tractor pulling a trailer, it's usually a pintle on the tractor which is run through a clevice on the trailer. For road going vehicles, the fifth wheel trailers are often used, where a trailer kingpin will be inserted and locked into a fifth wheel on the vehicle. In the case of an 18 wheeler, air lines will connect from the tractor to the trailer to operate the trailer's air brake system.