Tailwheel (conventional) landing gear is an aircraft undercarriage consisting of two main wheels forward of the centre of gravity and a small wheel or skid to support the tail.
The term taildragger is aviation jargon for an aircraft with a conventional undercarriage.
To decrease drag in flight some undercarriages retract into the wings and/or fuselage with wheels flush against the surface or concealed behind doors.
Some aircraft with retractable conventional landing gear have a fixed tailwheel, retraction the maingear only.