The holographic principle in its narrow sense is that if you have a "bulk" on which a type of physics called Conformal Field Theory takes place, and the boiundary of this bulk has the geometric shape of an anti-deSitter space, then the geometric (Einstein) dynamics on the boundary encodes all the information there can be about the physics of the conformal field theory on the bulk; this was shown by Maldacena and he conjectured that the universe works that way; we live on the four dimensional ant-deSitter space (approximately) and our physics is the "shadow" of physics in the bulk. Lisa Randall's recent book "Warped Passages" describes her extension of Maldacena's idea. Maldacena's idea is known as the AdS/CFT conjecture, and goes by the nickname holographic conjecture, because the boundary is "like a hologram" in containing all the information about a higher dimensional reality.
Maldacena's conjecture was inspired by the properties of a black hole; all the information that a
black hole shows to the world is expressed on its boundary, the event horizon. For example the entropy of the black hole is proportional to the area of the horizon, not to the volume it encloses.
Padmanabhan has just this week published a paper entitled "Dark Energy, the Mystery of the Millenium" in which he applies the thermodynamics of horizons, and the information encoding of three dimensional volumes by two dimensional horizons, to cosmology.