Chemical reaction

A reaction is a chemical change which forms new substances.
Example of a reaction is as follows: H2 + 1/2 O2 gives H2O i.e, water.

Reversible reaction: In a reversible reaction, the products of the reaction can be converted back to its reactants.
An example is given below:
2NaCl + CaCO3 gives Na2CO3 + CaCl2

Equilibrium reaction:It is a state of balance in which two opposing reversible chemical reactions proceed at constant equal rates with no net change in the system.

You can get help on equilibrium reaction with examples of different reactions online help websites.