A lightning conductor (United Kingdom) or lightning rod (United States) protects a building from lightning. It is designed to be struck instead of the building, so that it can channel the lightning safely down to the ground. Because the rod does not itself draw the lightning, it is mounted at the highest point of the building where it is more likely to be struck instead of the buliding.
The rod is only one component of the lightning protection system. In its simplest form, the copper or aluminum rod guides the lightning to a thick low-resistance wire, which connects to a conductive grid buried in the ground. Most lightning protection systems will have several alternate paths which the lightning can follow, and may also be built with surge interrupters and other protective devices. A large building protected by a lightning protection system will have several rods attached to the roof at regular intervals, or the rods may be replaced by a metal strip.
The charges which can be built up by thunderclouds are massive. A lightning protection system has to be able to handle surges of up to 120 kiloamperes, voltage up to one billion volts, and temperatures which can melt glass. Positively charged lightning, which comes from the top of the cloud, can be up to ten times more powerful.
Sometimes there will be a glow around the rod of a lightning protection system. This is caused by the ionization of the air around it. It does not mean that a lightning strike is imminent. On ships, such a glow is called St. Elmo's fire.
Today, all tall structures are built with lightning rods. Lower buildings around a skyscraper or tower are protected by the tower, since lightning is much more likely to strike the taller structure, but they are not immune to lightning strike. For example, the Empire State Building is struck by lightning an average of 23 times a year. Tall trees can protect a building in the same way as a nearby tower.
If you are standing outside during a thunderstorm, you have a greatly increased chance of being struck by lightning. Your wet skin will act as the lightning conductor carrying the charge to the ground. Unlike other types of electrical shock, lightning will follow the outside of your skin to the ground, rather than traveling through your body. Any metal you are carrying will further increase your chances of being struck. In one recent case, lightning struck an iPod and followed its wires up to the earphones, then down along the skin to the ground. If you are among the 2/3 of people who survive a lightning strike, you are likely to be left with permanent sensory and nerve damage.
To reduce your chances of being struck, stay indoors during electrical storms. Otherwise avoid tall objects, such as trees, and open areas where you are the tallest object. Also don't carry anything long and metallic, such as a golf club. Do you really want to be carrying your own personal lightning rod?