A popularizer is someone who is known not for original work in a field, but for publicizing or explaining the work of other people to a general audience. Examples:
Henry Hazlitt (
economics);
Carl Sagan (
astronomy);
Roger Ebert (
film);&Sister Wendy Beckett (
painting). It's a valuable occupation since many who are experts in their fields lack either the desire or the ability to communicate with a nonexpert audience.