Carbon. Rule of thumb: the bigger the filter (in volume)the longer it will last and the better the flow rate. A simple chlorine test kit will tell you when carbon is exhausted and needs to be replaced.

Andy c is correct. You'll be using some version of an activated charcoal stack.

Believe they are available separately but usually in combination with a particulate filter and timed backwash device. Several water softener vendors also include them as components of their softening units. I have Kinetico's version. Kinetico's installer advised that at my chlorine level (very faintly noticeable by smell if you concentrate while sniffing) the charcoal modules would probably last about five years and will cost several hundred dollars to replace when the time comes. Kinetico is always on the high end. Suspect almost any other vendor would be less either by a little or a lot. In any event, whatever you get WILL require replacement at some interval.