Fluoride is often added to municipal water in an unnatural form as sodium fluoride, sodium fluorosilicate, or fluorosilicic acid. Sodium fluoride is the most widespread type, but it is significantly more toxic than calcium fluoride, which is a natural form (source).
Although some water filters will remove the majority of fluoride, most will only reduce it. In comparison, distillation will remove all fluoride — leaving you with pure, safe drinking water.