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.