Dolby Digital is often referred to as a 5.1 channel surround system. However, it must be noted that term "Dolby Digital" refers to the digital encoding of the audio signal, not how many channels it has. In other words, Dolby Digital can be Monophonic, 2-channel, 4-channel, 5.1 channels, or 6.1 channels. However, in its most common applications, Dolby Digital 5.1 and 6.1 are often referred to as just Dolby Digital.
For details and technical information on Dolby Digital, check out the Dolby Labs Website.