What is the difference between “make a difference” and “make the difference”?

Make a difference means to have an effect.

Make the difference means to be the deciding factor between success and failure, or between two outcomes.

Examples:

• Drinking more water can make a difference to your health.

• One good example can make the difference between a confusing answer and a clear answer.

• His experience made the difference in the final result.

Use “make a difference” for general impact.
Use “make the difference” when something is the Important.