Semiconductors are materials that would be electrical insulators at zero kelvin, but at room temperature are merely poor conductors. A thorough understanding of semiconductor theory requires considerable effort in developing a background in physics, however, a basic picture can be developed with a little thought.
All normal state matter is made up of protons, neutrons, and electrons organized into atoms, molecules, and ionic compounds. (We'll neglect the quarks and gluons for this discussion as they do not directly affect the properties of anything large enough to pick up on the end of a needle.) The protons and the electrons are attracted to one another through the electric force. Gravity is too weak to have a measurable effect on the properties we are concerned with here. The strength with which the electrons are held to the protons is inversely proportional to the distance from the protons said electrons reside. The farther away, the less the holding, or binding strength.
In metals (like copper, Cu; gold, Au; and silver, Ag) the binding strength of a small fraction of the electrons is quite small. It is so small in fact the the outermost electrons on any given atom feel an equal pull from the next door neighbor. What this means is these electrons basically behave like a gas confined to some container. These electrons are free to move about the metal, and account for the various properties of the metal such as thermal conductivity and electrical conductivity.
In insulators (like table salt, NaCl; diamond; and most plastics) all of the electrons are tightly bound to the nearest atom. None of the electrons are free to wander through the material. This accounts for the lack of electrical conductivity and generally* poor thermal conductivity in insulators.
In semiconductors the outer electrons are bound to the nearest atom, but they are not held as tightly. A little bit of extra energy (from an increased temperature, for example) will result in a small number of the outer electrons getting free and being available to carry electrical current.
In the physics vernacular, metals have an electron energy band cut by the Fermi energy. Insulators have all of their bands far from the Fermi energy. Another way of saying this is that the Fermi energy lies in a gap. Semiconductors have their bands close to the Fermi energy, but not crossing it; they have a narrow gap. Room temperature thermal energy is sufficient to give semiconductors their small conductivity.
*Most insulators are poor conductors of heat. Diamond is a notable counter example, as it has the highest thermal conductivity of any natural substance, but zero electrical conductivity.