A toy line that was launched in 1984, robotic humanoid characters that could transform into vehicles and other objects such as semis or boomboxes. Their slogan was "more than meets the eye".
A TV show was launched the same year. The show featured pretty nifty writing for its time. Revolved around battling robot races from the planet Cybertron, the Autobots (good guys) and Decepticons (bad guys). They eventually crash-landed on earth and continued their battle for objects called "Energon Cubes", which were the power source for all transformers. The Autobots, led by Optimus Prime, were aided by human teenager Spike and his father Sparkplug (yep, those were their names) in their many battles against the Decepticons, led by Megatron. Your basic classic story of good vs. evil.
Also spawned an animated feature in the late 80s, featuring the death of Optimus Prime, and his resurrection in season 3.
Spawned various spin-offs such as "Beast Wars", "Transformers Armada", and "Transformers: Cybertron". But the best is the original.
A live-action feature film is coming in July 2007, under the direction of Michael Bay
, which, depending on your taste in movies, could go either way.