Activated charcoal is charcoal that has been treated with oxygen to open up millions of tiny pores between the carbon atoms. Activated charcoal is good at trapping other carbon-based impurities ("organic" chemicals), as well as things like chlorine. Many other chemicals are not attracted to carbon at all -- sodium, nitrates, etc. -- so they pass right through. This means that an activated charcoal filter will remove certain impurities while ignoring others.
A activated carbon is also called activated charcoal, activated coal, or either carbo activatus, it is some sort of form of carbon processed to be riddled with small, or low-volume pore that would increase the surface area available for aborption or chemical reactions. The meaning of activated is sometimes substituted with active.