What is enzyme, a substrate and competitive enzyme inhibition?

Enzyme: A biomolecule that speed up the rate of chemical reactions without being used up.

Substrate: A substance or chemical that enter the chemical reaction and is being converted into a new substance  (product).

Competitive enzyme inhibition: Inhibition of enzyme's activity by binding of inhibitor to substrate binding site of the enzyme.

Explanation:

Enzymes are the biomolecules that serve to increase the rate of reactions by lowering down the required activation energy. The enzyme is never used up during reactions.  

Substrates are the chemicals that undergo a chemical change and produce products.  

For example, Glucose is the substrate for hexokinase enzyme and is converted into glucose 6 phosphate (the product).

When the inhibitor competes with the substrate for the binding site on the enzyme and does not allow the substrate to bind to the enzyme, the process is called competitive enzyme inhibition. It can be overcome by increasing the concentration of substrate in the system.