A reflection nebula is a cloud of interstellar dust that becomes visible due to the light from nearby stars reflecting off its particles. Unlike emission nebulae, reflection nebulae do not emit their own light but shine by scattering the light of nearby stars.
Source: science.nasa.gov
21/01/2003

In the vast Orion Molecular Cloud complex, several bright blue nebulas are particularly apparent. Pictured above are two of the most prominent reflection nebulas - dust clouds lit by the reflecting light of bright embedded stars. The more famous nebula is M78, on the upper right, cataloged over 200 years ago. On the lower left is the lesser known NGC 2071. Astronomers continue to study these reflection nebulas to better understand how interior stars form. The Orion complex lies about 1500 light-years distant, contains the Orion and Horsehead nebulas, and covers much of the constellation of Orion.