A limb is usually means an appendage on something.
Limbs are used in most animals for locomotion, such as walking, running or climbing. Some animals can use their front limbs (or upper limbs in humans) to carry and manipulate objects. Other animals can use their hind limbs for manipulation as well.
In the human body, the upper and lower limbs are commonly called the arm and the leg. The lower limb is specialised for walking, and running to change location. The upper limb is very mobile allowing us to reach at a wide range of distances and angles.
See also: orthosis