Actually, that's not the symbols. A pentagram is simply the point-up star. A pentacle is the star within a circle. (Wicca doesn't use the point-down star - not on the whole. Individual traditions might, but that's up to them). As I learned it, the pentragram was borrowed form the pythagorean school of thought and it represents all the different aspects of the universe - being the perfect combination of a masculine (even, 2) and feminine (odd, 3) number, with 2 representing the two gods and 3 representing the three interests for living beings. Because of that it represents everything spiritual and physical, and is usually thought to be protective. The pentacle adds the circle to add to the idea of unity (as zero or a circle shape represents complete unity). By focussing on individual aspects of unity the pentacle is often associated with the five elements wiccan elements (water, fire, air, earth and spirit), as well as the five senses. The circle can also be used as a sort of diagram to represent the directions used for circle-casting in ritual, but I don't think that's a huge part of it. At least in my school of thought, they represent very similar things, however the pentacle is more associated with wicca and also has a stronger focus on unity and harmony. In ritual typically you'll use a pentacle. A pentagram may be used in situations where a pentacle just wouldn't work (maybe you're making the shape of a star with your body or something, idk) and will often represent something very similar or almost the same.