Asteroids are characterized based on orbit, size and composition.
Apollo - Earth crossing Near Earth Asteroid (NEA) with semi-major axis larger than the Earth's Amor - Earth approaching asteroid with orbits outside of Earth but inside the orbit of Mars Aten - Earth crossing asteroid with semi-major axis smaller than Earth's Atira - Orbit is contained inside of Earth's orbit Determining the Size of an Asteroid Radar - Radio waves emitted from large antennas bounce off an asteroid and are reflected back. Infrared Light - Infrared is more accurate than visible light because a small highly reflective asteroid can look the same size as larger less reflective asteroids.
SMASS Classification of asteroid types are based on color and spectral shape
C-Group are made up mostly of carbon and comprise 75% of known asteroids. S-Group are made up mostly of silicates, 17% of known asteroids. X-Group are metallic asteroid. D-Type asteroids are composed of organic rich silicates, carbon and possibly water ice and are found orbiting the outer asteroid belt and further. V-Type asteroids are large asteroids similar to Vesta and make up about 6% of the main belt. Additional types include T, Ld and O