Absolute and relative magnitude

The brightness of any celestial body (stars, asteroids, planets, etc) is measured by a quantity called magnitude.
The classification of stars of different magnitudes is made first of all, on an historically basis: the stars cataloged by Ptolemy (2d cent. A.D.), all visible with the unaided eye, were ranked on a brightness scale such that the brightest stars were of 1st magnitude and the dimmest stars were of 6th magnitude.
The modern magnitude scale was placed on a precise basis by N. R. Pogson (1856). It was found by photometric measurements that stars of the 1st magnitude were about 100 times as bright as stars of the 6th magnitude, 5 magnitudes lower. For this reason, Pogson defined a mathematical, exponential law to formalize the magnitude's scale. This modern scale allows a precise expression of a star's relative brightness and extends to both extremely bright and very dim objects.
The measurable brightness of any celestial object depends on many parameters such as the object's size and the distance from the observer (a candle very near you is much brighter than a very far -and very bright- star!). For this reason, the brightness of any celestial body, measured directly as you can see it, is also called relative magnitude. Since all the objects in the solar system are moving (and changing), the relative magnitude of an object changes in time.
It is therefore necessary to define an absolute magnitude for every class of objects (which can be asteroid, stars etc). An absolute magnitude is a quantity which measures a brightness independent of the distance. Normally, it indicates the magnitude the object would have if it were 1 AU from the Earth. Absolute magnitude is a measure of the intrinsic luminosity of the star, i.e., its true brightness.
This absolute magnitude can be defined in different photometric systems. Since in modern times magnitudes are measured with photometers and electronic detectors, which may be more sensitive to light at one wavelength than at another wavelength, it is necessary to specify the method and the filter used when comparing two or more magnitudes (click here to know more about the different wavelengths of light). The magnitude usually referred to is the visual, or photovisual, magnitude, measured with a photometer.