Landing gear is the undercarriage of an aircraft or spacecraft that is used for taxiing, takeoff or landing. For aircraft it is generally needed for both. It was also formerly called alighting gear by some manufacturers, such as the Glenn L. Martin Company. For aircraft, Stinton makes the terminology distinction undercarriage (British) = landing gear (US).
The retractable main landing gear of a Boeing 747
Conventional/taildragger Piper Cub
Tricycle Cessna 152
Bicycle AV-8B Harrier
Taxiing is the movement of an aircraft on the ground, under its own power, in contrast to towing or pushback where the aircraft is moved by a tug. The aircraft usually moves on wheels, but the term also includes aircraft with skis or floats.
Jet airliners taxiing
A privately owned Sea Vixen taxis back from an air show flight, with wings folding as it moves.
A Eurocopter EC120B hover-taxis