An S-type asteroid is an asteroid that is mainly silicaceous, or made of a lot of silicon. This is why it is called an S-type. About 17% of all asteroids are this type of asteroid, meaning that they are the second most common after C-type asteroids.
S-type asteroids are the most common asteroid in the inner main belt, but become very rare farther out in space. The biggest asteroid of this type is 15 Eunomia.