Pentacles and pentagrams are both intended as talismans of protection. In modern times, these protective insignias have become tokens of representation, but that is not how it was intended.
A pentacle is a five-pointed star, inside a circle, with two points down and one point up. When properly cast, it will protect those inside it from what is outside the sacred space.
Today, the Pentacle is a sacred icon of Paganism, Neo-paganism, and Wicca. They are worn either openly or covertly to represent the various religious group to which the bearer belongs. Faerie Wicca practitioners will often wear a pentacle bearing a faerie. Those in Asatru-Wicca will sometimes wear a pentacle with Thors hammer emblazened thereupon, and so forth.
Each of the five tips represents a sacred element. There is much debate as to which tip represents what element, so I always suggest using the elements in your world as the guide posts. If you live near a volcano, the direction of the volcano should be designated your fire elemental post, the ocean as water, and so on.
The Pentacle has received a bad reputation thanks to much of the horror industry. It is wrongly associated with Satanism; a Christian heresy. Pagans are not Christian; therefore, Satanism falls into one of the hundreds of Christian branches, and a pentacle has nothing to do with that or with them.
Pentagrams, on the other hand, do. A pentagram is designed to protect the practitioner from what is inside the circle. What is called, summoned, conjured, etc. is brought into the circle, while the practitioner(s) stand outside. It is intended to hold the entity within its borders. That is the easy definition, but I must say that if I were calling something big and nasty, I would want a pentagram there on the floor and I would pray it's not strong enough to break my Magick. But I don't mess with those, so happy summoning.
Today, the Pentagram is often associated with Satanism for the very reason I've detailed above. And rightly so. It is basically the Pagan version of the inverted Christian cross; it's a blasphemy. It is a complete bastardization of who and what a White Witch is and does.