Undercarriage or landing gear is the structure that supports an aircraft on the ground and allows it to move on the ground. Wheels are the most common used but other types of undercarriage might also be used or combined depending on the terrain. For example: a plane in the arctic regions would likely use skids and a waterplane would use floats.
Landing gear is most commonly wheels equipped with shock absorbers. Some aircraft are equipped with skis for snow, floats for water and skids/pontoons for helicopters.
Retractable Landing Gear
To decrease drag in flight, some undercarriages retract into the wings and/or fuselage with wheels flush against the surface or concealed behind doors, allowing fluent aerodynamics.