Electrical insulators are materials with a high resistivity (resistivity is a property of the material) so they can make objects with a high resistance. This allows insulators to prevent electric current from flowing where it's not wanted.
Insulators are useful for coating wires, or acting as dielectrics in capacitors. An insulator (such as plastic, rubber, or glass) can have 1020x the resistivity of a metal like copper. Often times these insulators are colour-coded to make it easy to tell what function the wire inside is serving, see figure 1 for an example.
Air (like in the atmosphere) is actually an excellent electrical insulator. This means that electricity can be sent through a conductor and it won't jump through the air. Air does have an upper limit to the voltage it can handle, which is called the average breakdown voltage (to learn more about average breakdown voltage, please see all about circuits. This breakdown voltage is related to lightning.
To learn more about the physics of insulators please see hyperphysics.
For further information please see the related pages below: