It has been established that a lightsaber functions by focusing energy (of some kind) through a central lens (positively charged). When the energy beam reaches the apex of its path, it curves back sharply into the negatively charged outer ring.
It's been asked what contains the beam and gives it substance. I propose that there is an extremely tight and powerful magnetic field around and through the lightsaber blade. The gradient of this magnetic field would be such that bits of iron wouldn't fly towards it from all sides when it's turned on, but it would provide a seemingly solid resistance when struck with another lightsaber because of the negative charges on the outside of each blade. Of course, it would pass through almost anything else quite easily. This powerful magnetic field may also be what protects those that it touches from being electrocuted, simply burned.
The magnetic field would likely be created by the energy of the lightersaber blade when it's turned on and focused by the crystals within the handle.
This explains the sparks and crackling when two lightsabers collide, as electrons are forced between the two blades as the fields weaken and either into the apex of the blade or back into the base.
While it's easy to say 'electron' and 'magnetic field', it doesn't completely explain certain effects like cutting through metal and the fact that no one in the fiction has ever made a lightsaber with an opposite polarity (but why would you want someone's lightsaber to stick to yours? Well, only if it turns off if it's not in his or her hand). It may be that it's something more exotic, like a stretched meson (as proposed in another node), or the field may be a monopole magnetic field (currently impossible; magnets usually have both a North and a South pole).
A monopole magnet may do the job of containment just as well, if it's a postive (North) pole. The positive monopole around the negativly-charged blade surface might give the lightsaber that extra bit of power when it comes in contact with another lightsaber, since more energy could leap across and through the other saber, while retaining the nearly solid resistance of two similar poles against each other.