A SELRES (Selected Reserve) is the traditional "weekend warrior," historically one weekend a month and 12 to 17 days of active duty per year. Most guys do a bit more; some do lots more.
FTS (Full Time Support) is a term that describes personnel who are primarily focused on administering, managing, and training reservists and reserve programs. But again, things have changed considerably. While many work in Reserve Centers or at Reserve Squadrons providing day-to-day continuity, a growing number work on major Active Duty staffs (Second Fleet, U.S. Fleet Forces, Joint Forces Command, etc) as the Reserve Liaison Officer to the staff. Many also are assigned to active ships and squadrons and deploy with the rest of the unit.
A SELRES can request to transition to FTS, just like an Active Duty guy can and they have selection boards annually.
Sometimes the only way to tell the difference is comparing designators. An active duty Naval Aviator will have designator 1310; a SelRes will have 1315; and an FTS will be 1317. A SelRes retains the XXX5 designator, even if mobilized or recalled to active duty, unless he / she actually petitions to accept an active duty commission again.