An RNA Polymerase is an enzyme that transcribes a DNA strand from 5' to 3' to form an RNA chain. As such, RNA polymerase is a critical enzyme in the central dogma where DNA is transcribed to form RNA and RNA is translated to form proteins. An RNA polymerase normally begins transcribing DNA at particular DNA sequences. These DNA sequences are known as promoters.
To read more about specific RNA Polymerases, see the following pages -