A longbow is a long, hand-drawn bow designed to shoot arrows at a distance and which is at least as long as the archer is tall; modern longbows are typically 6' to 6'6" in length, and some recovered medieval examples are over 7' long. It was made famous from its use in medieval England. It is a common misconception that the longbow was a uniquely British weapon, as similar bows have been found in Scandanavian peat bogs and are used for elephant hunting by a number of African tribes. The English were first educated in the effects of archery by the Welsh during Edward I's campaigns in Wales, and subsequently developed the Welsh self-bows into a longer, much more powerful weapon, typically made of yew.
What is unique is the way it was used en masse and with tactical brilliance by English commanders during the 14th and into the 15th centuries, and the discipline and training of the archers - an Anglo-Welsh yeoman archer of the time was trained to loose an arrow, generally 32" long, up to 250 yards, with a good deal of accuracy, at a rate of between 6 and 10 arrows a minute. This is a rate of fire not exceeded until the first repeating rifles of the 19th century.
It should be noted that bow quality changed over time and all longbows were not the same. Recovered longbows from the time of Henry VIII have draw weights ranging from 120-200lb, far beyond the capabilities of most modern archers or bowhunters, but by this time there were frequent complaints that England's archers were far weaker than they had been during the Hundred Years' War. While the longbow lacked range compared to Asian composite bows it was capable of launching a much heavier arrow, which gave it a far greater ability to penetrate armor. In fact, at short range, an arrow from a longbow could penetrate 4 inches of seasoned oak. In addition, its simple design made it much faster to shoot. The longbow proved to be a devastating weapon in the hands of English archers, who demonstrated enormous success against armies with significantly greater numbers of troops. Good examples of this were the Battle of Crecy and the Battle of Agincourt during the Hundred Years War where heavily armored French knights were unable to maneuver effectively and suffered prolonged exposure to withering longbow fire.