The apothem is one leg of a right triangle and the radius is the hypotenuse. A hypotenuse is always longer than either of the two legs.
The radius of a regular polygon is the distance from the center to any vertex. It will be the same for any vertex. The radius is also the radius of the polygon's circumcircle, which is the circle that passes through every vertex. In this role, it is sometimes called the circumradius.
The apothem is also the radius of the incircle of the polygon. For a polygon of n sides, there are n possible apothems, all the same length of course. The word apothem can refer to the line itself, or the length of that line.
Find the apothem of a regular polygon or circumradius of the incircle using this free online calculator.