Lightning (which is shorthand for Hash Locked Bi-Directional Payment Channels) is a trust-less net settlement system built by leveraging the smart contracting capabilities of bitcoin.<p>Such a system enables transactions to occur between a limited set of participants without the requirement that the entire world know about them.<p>Avoiding telling the entire world reduces the cost of transaction processing by many orders of magnitude (something on the order of 1,000,000 times cheaper).