What is the different between a Bonus versus a retention bonus

For tax purposes, nothing...and they both are the same as any income from your employer for tax. Definitionally - Bonus is normally for achieiving certain results personally or as part of the organization - (like having the best attendence gets a bonus, or everyone in the department gets a bonus for getting the big project done on time). It is intended that the awarding of it and the amount would be discretionary on the employers part. A retention bonus is for your promise to stay on for at least some time or specific event. For example, the company has been sold and as what you do needs to be done for some time and tranasferred to the new owners staff, you will be given a bonus if stay until the XYZ system is converted to theirs.