Supports an aeroplane on the ground and absorbs vertical shocks on take off and particularly, landing.
The main part is a shock absorbing strut, comprising a piston and cylinder, like a very large bicycle pump.
The cylinder is filled with high pressure nitrogen and hydraulic oil.
The Nitrogen acts as a spring and resists the piston being forced into the cylinder, contracting and expanding in response to external forces.
Movement of the piston in the cylinder forces the hydraulic oil to flow through restricting orifices and slow the movement, removing any violent changes and making the movements comfortable for the passengers.
Typically, there are two or more large units (Main Landing Gears) in the middle of the aircraft, which are capable of supporting the full weight of the Aircraft; and a smaller one; at the front (Nose Landing Gear) basically intended to support the aircraft with its cabin floor horizontal whilst on the ground.
Some aircraft like the 747 has 5 gears, two on wing, two body gears and a nose gear, making them distinctive in appearance.
B777 main landing gear gears are also easy to spot as they have six wheels whereas most other main gears have 4 wheels each.