Landing is the last part of a flight. Hitting the ground too hard is prevented by wings (including rotor wings), a parachute or rockets or a vertically directed jet engine; in the case of a balloon the buoyancy is slightly decreased for a soft landing.
For aircraft or birds, landing is generally accomplished by trading airspeed for lift. During the flare, they will reduce the rate of descent and rotate into a landing attitude shortly before reaching the ground. The nose continues to rise which increases the angle of attack of the wings and therefore the amount of lift generated, as the airspeed gradually reduces.
In a perfect touchdown, assuming there is no crosswind, contact with the ground is made just as the forward speed is reduced to the point where there is no longer sufficient lift to remain aloft.
During landing, the ground effect becomes significant for aircraft. This tends to make the aircraft "keep flying" when it ordinarly would not (at higher altitudes) and therefore to extend the distance required to land.