Carbon is a filter media that can be used, but today it's considered optional and somewhat controversial.
Carbon, especially when it's not rinsed completely clean, is, by some, considered a major factor in hole in the head disease, and lateral line erosion. On the plus side, carbon is good for keeping the water clear. It also can provide a lot of surface area for bacteria, but then so do a lot of other media.
There are also other more modern media such as Purigen. It's more expensive, but it's rechargeable.
If you are going to use carbon, use the highest quality carbon you can obtain. It will be more expensive, but less likely to cause problems.
My personal preference is to use carbon only when I have a specific issue to resolve, for example, something big died and released some toxins. I consider the full time use of carbon optional, but I wouldn't call someone that uses it all the time wrong.