Once Franklin had an understanding of the behavior of electricity, he set about to protect houses from the destructive forces of lightning. A lightning rod, simply, is a rod attached to the top of a building, connected to the ground through a wire. The electric charge from lightning strikes the rod and the charge is conducted harmlessly into the ground. This protects houses from burning down and people from electrocution.
In a 1753 edition of Poor Richard's Almanack, Franklin published this description of his invention:
It has pleased God in his goodness to mankind, at length to discover to them the means of securing their habitations and other buildings from mischief by thunder and lightning. The method is this: Provide a small iron rod (it may be made of the rod-iron used by the nailers) but of such a length, that one end being three or four feet in the moist ground, the other may be six or eight feet above the highest part of the building. To the upper end of the rod fasten about a foot of brass wire, the size of a common knitting-needle, sharpened to a fine point; the rod may be secured to the house by a few small staples. If the house or barn be long, there may be a rod and point at each end, and a middling wire along the ridge from one to the other. A house thus furnished will not be damaged by lightning, it being attracted by the points, and passing thro the metal into the ground without hurting any thing. Vessels also, having a sharp point rod fix'd on the top of their masts, with a wire from the foot of the rod reaching down, round one of the shrouds, to the water, will not be hurt by lightning.