It is a common experience that lifting an object in water is easier than lifting it in air. It is because of the difference in the upward forces exerted by these fluids on the object.
The upward force, which acts on an object when submerged in a fluid, is known as buoyant force. The nature of buoyant force that acts on objects placed inside a fluid was discovered by Archimedes. Based on his observations, he enunciated a law now known as Archimedes principle. It state that when an object is submerged partially or fully in a fluid, the magnitude of the buoyant force on it is always equal to the weight of the fluid displaced by the object.
Example of buoyant force is the motion of hot air balloon. Since hot air has less density than cold air, a net upward buoyant force on the balloon makes it to float.
A piece of wood floats on the surface of water. One of the forces is due to gravitational force, which pulls it downwards. However, the displaced water exerts buoyant force which acts upwards. These forces balance each other in equilibrium state and the object is then said to be floating on water. It means that a floating body displaces the fluid equal to its own weight.