What is a Terminal Radar Service Area (TRSA) and what are its operational requirements?
A good explanation of a Terminal Radar Service Area (TRSA) can be found in the Aeronautical Information Manual 3-5-6.


  TRSAs were originally established
  as part of the Terminal Radar Program at
  selected airports. TRSAs were never controlled
  airspace from a regulatory standpoint because the
  establishment of TRSAs was never subject to the
  rulemaking process; consequently, TRSAs are not
  contained in 14 CFR Part 71 nor are there any TRSA
  operating rules in 14 CFR Part 91. Part of the Airport
  Radar Service Area (ARSA) program was to
  eventually replace all TRSAs. However, the ARSA
  requirements became relatively stringent and it was
  subsequently decided that TRSAs would have to
  meet ARSA criteria before they would be converted.
  TRSAs do not fit into any of the U.S. airspace classes;
  therefore, they will continue to be non−Part 71
  airspace areas where participating pilots can receive
  additional radar services which have been redefined
  as TRSA Service.


These airports can't be redrawn as class B or class C airspace so they became TRSAs. The airspace at the airport is class D and the airspace in the TRSA is usually class E. The operational requirements are no different than any other class E or class D airspace, but aircraft are encouraged to avail themselves and participate in the TRSA when inside its bounds.

It should also be noted that many TRSAs have their own approach control.