For the purpose of the ease of moving electrons, matters are categorized as conductors, semi-conductors, and insulators. Conductors are materials like silver, copper and gold, among many things. Semi-conductors are materials like silicone and germanium. Insulators are materials like glass and ceramic. It is important to know these materials because they are often used to make electronic components.
Conductors have atoms that only have 3 or less electrons in its outer shell. This makes it easier for electrons to flow.
Semi-conductors have atoms that have 4 electrons in its outer shell. This creates resistance against electron flow.
Insulators have atoms that have 5 or more electrons in its outer shell. This does not allow electrons to flow.
Mark Lauckner adds "Did you know that glass and ceramic materials do indeed conduct electricity, beginning at around 1000f. I wonder what happens to the electrons to enable this? Some modern glass factories actually use the molten glass as the heating element in electric furnaces... "