The use of activated carbon is pretty straightforward: it boils down to combining infinitesimal grains of carbon with contaminated things you want decontaminated -- water, air, metals or the human body.
The great thing about carbon (other than giving birth to diamonds) is that it naturally binds to organic toxins. So if you add some carbon to contaminated water, the icky particles will cling to it -- then you scoop it out, leaving clean, drinkable H2O behind. If you’ve ever had water from a Brita filter, you’ve benefited from activated carbon.
So what is activated carbon? Carbon is "activated" by steaming it at temperatures up to 1800 degrees Fahrenheit, creating lots of tiny pores and pockets. The powdered form takes this a step further, grinding the carbon into granules that are less than a millimeter in size.
The vital term is surface area. On a brick of carbon, only the molecules on the outside are available to bind to toxins. But if it's full of nooks and crannies, many more molecules are exposed and can be put to good use. In powdered form, that number skyrockets: One gram of powdered activated carbon can have a surface area of 500 square meters.