Materials that present a very high resistance to the flow of electricity are known as insulators. These materials conduct electricity poorly because their atoms (or molecules) hold tightly onto all of their electrons, preventing electrons from roaming freely through the material. Notice in the following picture that the electrons are rigidly bound:

Source: Silicon covalent bond, Khan Muhammad, Wikimedia

Because of this, insulators generally do not look shiny, they do not conduct heat well, and they are often brittle. Examples of good insulators are wood, plastic, rubber, and glass.