Light single-prop passenger aircraft with some kind of pod on the belly and maybe some instrumentation on the wings.
It did not appear on the ADSB plotting sites.
It's a Cessna 208B Grand Caravan. The pod on the bottom is for luggage I believe. There are pitot/static tubes on each wing to measure airspeed and trim and a weather radar bulb on the right wing, just inboard of the pitot/static tube.
Pilots do circles for many reasons, usually related to training reps. There may be a radio beacon the pilot must fly past in a certain path for instrument qualification and so they are practicing.
Another reason to fly circles around an airfield may be to practice takeoff and landing. When they do this, they call it flying traffic circles, because they have to pay attention to traffic and communicate their intentions to other flyers and the tower.