Dielectrics and insulators are the same thing essentially. They are similar in terms of the class of materials, but they can used in different contexts.
Insulators are non-metal objects that don't allow for charges to move properly. An example of an insulator is an electrical cord. The substance that surrounds an electric cord does not allow for the electrical charge to escape the wires. This is why you don't get electrocuted when you touch an electric cord. Anything that is a non-metal solid is an insulator. The opposite of an insulator is a conductor. A conductor can transfer electric charges.
Dielectrics are non-metal plates that are designed to increase capacitance. Dielectrics are only refered to as a non-metal layer in a capacitor. Capacitance is the ability or the measure of storing electrical charge. Dielectrics are used in reference to capactors. They increase capacitance by basically lowering the voltage. For example, if there is two parallel metal plates with a dielectric (non-metal plate) in between, the voltage will decrease because dielectric increase the capacitance (capacitance is inversely proportional to voltage)
Summary: Insulators and Dielectrics are both non-metal substances that don't allow charges to follow or conduct. However, dielectrics are used in reference to capacitors only whereas insulators are used in reference to non-metal solids (plastic, glass, etc.)