Computronium refers to material engineered to maximize its use as a computing substrate. It can refer both to advanced hypothetical materials engineered by nanotechnology on the molecular, atomic, or subatomic level, or to contemporary computing materials.
Currently our computational devices are relatively large, power-hungry, complex individual devices — microchips. This is a problem, because they are not flexible enough, they are expensive to design and manufacture (a new chip fab plant costs almost $10 billion).
To move to smaller, cheaper and more flexible design a transition to parallel processing is needed and the chip manufacturers are already making multi-core CPUs.
The next step is to make lots of smaller and smaller chips, the tiniest viable fragments, about a tenth of a millimeter or so, then literally sprinkle them into a viscous medium, and pour out computing by the pound or by the square inch. In this way you can paint a computer on your wall and if it's not powerful enough you put on another coat of computer. 
Bill Butera from MIT CBA developed a programming model where little code fragments hop from particle to particle, traveling around and self-organizing into a system that solves a problem. The vision is to change the computer from a monolithic box to a raw material that gets configured by instructions traveling through it.
Currently simple chips cost below 1$, but they are still traditional chips, meaning the installation is not yet completely trivial. However, they make it possible to gradually add more intelligence to the products (the process that will eventually lead to smart matter).
When the chips become small enough, it will allow a transition from chips to first real computronium (pourable computing above), similar to a transition from custom electronics to general-purpose programmable chips in the past. It's important to note, that gaining flexibility doesn't mean a loss in precision. A computronium "liquid" computer can be every bit as precise and reliable (if not more) than a traditional CPU.
Afterwards the improvement of computronium will be a gradual process, eventually using more and more advanced nanotechnology. The distinction between computing and acting/manufacturing devices will slowly be eliminated and nanorobots will most likely combine both these functions. Smart matter will also be active matter.
Wikipedia has an article on computronium that discusses advanced computronium of the long-term future.