Inertia can easily be described as the reluctance or resistance of a body to change its state of motion.
1. If a body is at rest, it will continue to remain at rest if an external force is not applied to change its state of rest.
2. If a body is in motion, it will continue to remain in motion if an external force is not applied to change its state of motion.
Example: When you push a book which is on a table, it stops after a while. This happens because of presence of external force i.e. friction which stops the book. If friction was absent, then the book would have continued to move till the end of the table.