Mass is both a property of a physical body and a measure of its resistance to acceleration (a change in its state of motion) when a net force is applied. An object's mass also determines the strength of its gravitational attraction to other bodies.
The basic SI unit of mass is the kilogram (kg). In physics, mass is not the same as weight, even though mass is often determined by measuring the object's weight using a spring scale, rather than balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force. (wikipedia)
The weight converter converts between different units of mass, such as pounds, kilograms and grams, tons and tonnes. Useful for when you're told a man's weight in weird mass units (cough) and you're left wondering just how heavy that man really is.
This tool was developed for programmers and testers who have the constant need to convert between different units of mass.