Polymer
Polymers are high molecular mass (103−107μ) substance which are also referred to as macromolecules, which are formed by joining of repeating structural units on a large scale.
Example - rubber, polyethene, nylon 6,6
Monomer
Polymers are made of many small molecules linked together. These molecules are linked via covalent bonds. The small molcules that come together to form polymers are called "monomers"
So, monomers are the simple and reactive molecules from which the polymers are prepared either by addition of condensation polymerisation.
Examples: Ethylene, vinyl chloride, Acrylonitrile, Isoprene.