Hey,

Below I have explained both the terms separately for you to understand easily.

Bonus Shares: As the name suggests bonus shares are the free shares that is given to the employee or the investor . Apart from your bought shares you are given free shares as a reward in place of extra pay.

Buyback share: As the name suggests it involves buying of the shares back by the company from the buyer by giving extra price from the market price for that particular share.

So free shares are referred as Bonus Shares whereas buying share with price more than market value is called as Buy back share.

Hope that helps.

Thank you.