HA! You probably thought this was going to be a concise description of Hypercard. FAT CHANCE! If you don't know what it is, I'm certainly not going to try to explain it!
However, if you use Hypercard regularly, and have a hard disk, you really should try out HyperStation* version 3.2.
If you develop Hypercard stacks for yourself or others, you should check out the Developer* Stack.