(wiki): Mass is a property of matter. There are two important physical properties which are referred to as a mass. The first is called gravitational mass or gravitational charge. That property shows the strength of gravitational interaction between two objects which have gravitational masses. If gravitational mass is zero, there will be no gravitational interaction. Currently there are no known objects with negative mass, although it is not proven that mass must be positive or zero. The second property is called inertial mass. That property shows how well the object changes its velocity if it interacts with another object. The easier (e.g. faster) the object changes its velocity the smaller the mass is. A.Einstein conjectured that these two masses are proportional to each other. The conjecture is not proven till present time. The most sophisticated experiments were able to show that if there is a difference between these two properties it should be extremely small, less than approximately 10-11 of the mass. For practical purposes one can safely assume that these two masses are the same, but the problem still has tremendous interest in theoretical physics. If the masses are not the same the acceleration of objects with different masses in the same gravitational field will be different. Current knowledge that all objects accelerates near the Earth with the same acceleration is just approximation based on assumption that gravitational mass is proportional (or equal) to inertial mass.