In classic Wikipedia-voice, an atlatl is…
An atlatl (from Nahuatl ahtlatl [?ah.t?at?]; in English pronounced [???t?l??t??] or [??t?l??t??]) or spear-thrower is a tool that uses leverage to achieve greater velocity in spear-throwing, and includes a bearing surface which allows the user to temporarily store energy during the throw. [...] A well-made atlatl can readily achieve ranges of greater than 100 meters.
Atlatl Bob describes it more passionately:
…the Atlatl and Dart is the first true and natural weapons system of the human race, invented thousands of years before the Bow and Arrow and used longer by humans than any other weapon system yet developed. It was the Atlatl and Dart that first placed humans at the top of the food chain, ranking us above all other predators. It is, in fact, our natural ability to throw a projectile at prey that separates us from all other predators. Predators such as Lions, Tigers, and Bears run faster than humans and have sharp teeth and pointed claws, which we do not. Those are the attributes which make these predators successful. The Atlatl is the supreme expression of our natural ability to throw a projectile at a prey animal and is our advantage over all other predators, which makes us the supreme predator. And when humans used this weapon we did not take more from the environment than the environment could naturally replace itself. Thus, humans and nature, for thousands of years, were in balance with one another.