My theory - just made it up - a lightsaber is a hologram formed by phase conjugation of third-order non-linear vacuum polarization (chi-3). A fusion energy source emits gamma rays which form the hologram, and in turn the hologram shapes the beam, it's a feedback effect.

A lightsaber beam can block another light beam in the following way - the incoming beam acts as a wiggler for the lightsaber pump photons, which first get downshifted and then disintegrate into matter-antimatter pairs. The trick is to separate the matter from antimatter and channel them into the reactor. So firing at a lightsaber actually charges it.

The problem of course is conservation of momentum. To counteract the backwards jolt, a lightsaber creates a forward momentum impulse. This counteraction is not perfect, so Jedi are trained to maintain their balance in such situations.

When two lightsabers contact their beams are actually consumed by the other. That's why the Jedi/Sith can go on fighting forever, all the while throwing senate furniture at each other.