C-class
This is a photograph of 253 Mathilde, a C-class asteroid. C-class asteroids are by far the most common type of asteroid, making up an estimated 75% of all asteroids. That number could actually be much higher, however, as these asteroids have a very low albedo, reflecting approximately 3% of the light that strikes their surface, making them very hard to see. Their composition is thought to be similar to carbonaceous chondritic meteorites, and also to the Sun, minus hydrogen, helium, and other volatile elements.
S-class
Here we have an image of 951 Gaspra, an S-class asteroid. This class makes up about 17% of known asteroids, and they are made of almost equal parts metal and silicates. They are moderately bright, reflecting 10% to 20% of the light that strikes them.
M-class
This is a computer rendering of M-class asteroid 216 Kleopatra. It was made by radar imaging of the asteroid. M-class asteroids are sometimes, but not always, made of nickel and iron, and have albedos in the 10% to 20% range as well. Some consider them to be the iron cores of differentiated, or layered, asteroids that have survived an impact which destroyed the outer layers.
