Digital speakers are powered, or "active" loudspeakers that also include digital to analog conversion and other types of processing such as digital eq. With this type of speaker no external amplifier is needed, only line-level cables from a pre-amp.
The most famous brand of digital speakers is Meridian.
Another version of the digital speaker is a theoretical design that could result in more efficient amplifiers, but is impractical in the real world. Check out the Wikipedia page on these designs.