Basically, it's a geometric shape consisting of two interconnecting tetrahedrons. It's a three dimensional version of the Star of David. Yes, I think it's roots have something to do with Kabballah. The merkaba is large enough to fit a person inside. There are a lot of different versions and interpretations of how to activate one. Some say to spin the two tetrahedrons in opposite directions, some say the reverse of whatever that direction may be. Drunvalo Melchizedek instructs people to spin the merkaba in both directions simultaneously, but I've heard from someone here on BL that he had somehow gotten the spinning ratios wrong.
Drunvalo Melchizedek calls the MerKaBa "the most powerful shield known to man". In ordinary situations, especially conflicts between people, chi is easily transferred from person to person, but it seems a MerKaBa has the capability of "locking in" this chi and making sure it is only released if the bearer chooses to. That said, it's almost like the MerKaBa is a sentient organism, having it's own will, almost. For example, people say you can program your MerKaBa to perform a certain task just by meditating on it and telling it what to do. Furthermore, I've also heard it has the capability to heal people, both on a personal level as well as healing other people with it. I think it has a multitude of other functions to.