There are some things you hear about that can just boggle your mind. This is one of them.
Circuit bending is an art form unlike any other - it requires intelligence, creativity and a smidgeon of insanity.
Let's explain what it is...
What's it all about?
Ever had one of those kids toys that says things like "The Cow goes Moo"? Circuit bending is the process of turning that into "The Cow goes BZZZZZZzzzzzBZBEOgeOCEOOOOOooooooo".
It is finding conventional things such as toys and music making machines, usually cheap or second hand ones, opening them up and playing around with their internal circuits until they make sounds they weren't necessarily designed to make.
More specifically, this involves the circuit bender putting a jumper wire between two points on the circuit board and seeing what sound is made from the device's internal speaker. If it makes some awesome noise that they like, they can make it more permanent by soldering buttons or switches to connect those points, turning it into a modification or "mod" of the original device.

When did it start?
It has been going for as long as these toys and keyboard have been invented. There are always people who have messed with things in ways they weren't intended to be messed with... However, the first recorded circuit-bender, and the man who coined the term "circuit bending" is Reed Ghazala; a musician and experimental instrument builder.
The multi talented American started his circuit bending fascination by chance, when he accidentally short circuited a toy amplifier against a metal object in his desk, making a crazy sound. Instead of getting annoyed, he decided to make an art form out of it, and continues to promote the experimental phenomenon on his website. This dedication has earned him the title of "The Father of Circuit Bending" by Circuit Bending enthusiasts.

How did it get popular?
It started gaining popularity, however, past the year 2000 with events like The "XCOM Extreme Computing Festival" giving limelight to circuit benders and programmers who dared to design unusual and downright mental sounds from their existing toys.
More recently, with genres such as chiptune and glitch hop advocating the use of circuit-bent electronics in a more accessible and musical way, the art form has gained new levels of popularity. Artists use anything from the previously mentioned childrenÃs toys to Guitar Hero controllers to retro drum machines to create their signature sound and impress on stage.
Why is Circuit bending so crazily entertaining?
It's delving into the insides of a device you have no clue how it works and seeing what noises you can make. It's learning by trial and error how to make the most painful, hilarious and inspiring noises you can from devices that were not made to do so.
Some people go as far as to say it's a spiritual process - controlled chaos, bringing things into existence that should not exist. While that view is not for everyone, I think most people can appreciate the element of fun created by just messing around with toys and seeing what they can do.

How do I get started?
First things first - under no circumstances do you ever open up and attempt to circuit bend anything that is connected to the mains. You will die - and that would be a seriously stupid way to die.
A couple of techniques to get you started:
Looping
If you connect the output of the synth/sample circuit that makes the sound in a device to the input of the amp circuit, then the output of the amp circuit to an input on the synth circuit, then trigger the device, chances are you'll get some seriously obscure feedback-loop sounds!
Clock Bending
If your device has what is known as an "R/C" clock, you can make yourself a sweet pitch bend device! Just press on it gently (it should be a resistor near the synth chip) and you'll hear the pitch change as you do so!
Remember, the whole art form is based on experimentation - see what crazy sounds you can make with any old toys and synths you have lying around...
Whether it's the start of a new way of life or just good for a laugh is up to you!