Tastemarkers are individuals or small groups who are viewed as "making tastes" in addition to influencing groups of people. In order to make tastes or develop new tastes, these people will often be found in the arts, architecture, fashion or music businesses are these are very creative types of industries.
The label Arbiter of Cool will often be applied to these individuals or groups. Arbiter comes from the Latin arbiter which means "a witness or spectator". This leads to the use of arbiter as a "person who has the power of judging and determining." So factually the Arbiter of Cool is a person who has the power of judging and determining what is "Cool".
Marketing to these Arbiters of Cool or Tastemakers is called Tastemaker Marketing.