In some things, if you do something to it, like applying force, the molecular structure does not change. No new substances are formed. Such changes are called Physical Changes. Most of them are reversible.

When something is performed on a substance, sometimes new substances are formed, and its molecular structure becomes altered. Such changes are called Chemical Changes. Most of them are irreversible.

Paper: When you tear paper, it does not change. It's still paper. Therefore tearing of paper is a Physical Change.

Paper: When you burn paper, new substances like C02 and smoke are given out. New substances are formed. Therefore, burning paper is a Chemical Change.

A change that effects only the physical properties of a substance such as its size , shape or state the change is said to a PHYSICAL. The substance after the change is identical to the substance before the change. No new substance is formed. It is mostly reversible but it can also be irreversible.
FOR EXAMPLE: Changing of water into ice is a reversible physical change in which only the state is changed. Tearing of paper is also physical change but it's irreversible. 

A change that effects the chemical properties of an object is called a chemical change.The substance after change is totally different from the substance before the change. New substances are formed in a chemical change. It is mostly irreversible.
FOR EXAMPLE: Rusting of iron is a chemical change.