Pentacle

A Pentacle is a symbol of order with five points and five sides.

Description
Per Harry Dresden: it's a symbol of order with five points and five sides representing the forces of air, earth, water, fire, and spirit. It is contained within a circle, the points touching the outer ring. It represents the forces of magic bound within human control. Power balanced with restraint.

In the Series
In Small Favor, Karrin Murphy calls Harry to a crime scene in which a burning building had the whole front taken off. On the back wall of hte building, Harry finds a sigil painted in blood looking similar to a pentacle. Harry explains what a Pentacle is to Murphy and how a pentagram is different. At this point Harry is uncertain what it means since a symbol can have varrying meanings, but he's working on it.

Pentacle vs. Pentagram
Harry shows Murphy that the points of the star on his Pentacle touch the outer ring while the points of the star in the sigil they found on the wall go outside the circle. Harry says that it's uncomfortably like a combination of a pentacle and an anarchy symbol. — Magic unrestrained.

External References
Pentacle — Wikipedia

Pentagram — Wikipedia

Difference between a Pentacle and a Pentagram — Wiki Answers

Pentacle vs. Pentagram  — Yahoo Answers

Pentacle vs. Pentagram — Wiccan Information blog

Frequently Confused Symbols —Religious Tolerance Pentagon — Wikipedia