Carbon is the sixth element in the periodic table. It has a relative molecular mass of 12 even though other isotopes do exist. Carbon comes in 3 main allotropes graphite,diamond, and various fullerenes. Even though these are the same element they have very different properties.
Carbon is one of the world's most abundent elements, if you could imagine our lives without food, plastics, natural gas, petroleum etc then you would be half way on to understanding how important carbon is. In fact if it were not for carbon life itself would not be possible. Processes like respiration would not be possible, in fact the building blocks for the life (DNA) even contain carbon.