Semiconductors are materials that exhibit electrical behavior somewhere between that of insulators and that of conductors. Conductors present very low resistance to the flow of current, whereas insulators conduct very little current even when a large potential difference is applied. A semiconductor exhibits intermediate conductivity because it has more available charge carriers than an insulator but fewer than a conductor. Examples of semiconductors are silicon and germanium.
Why are semiconductors so important? Well, with the right amount of impurities and clever arrangement of materials, semiconductors can be used in the fabrication of resistors, capacitors, inductors, and—most importantly—transistors. It is because of semiconductors that we have incredibly complex technology such as computers and cell phones; these devices are based on integrated circuits, which can combine millions or even billions of transistors into one small package.