What is the difference between level shifter, voltage regulator and dc-dc converter?

What is the difference between level shifter, voltage regulator and dc-dc converter?

My understanding is:

• level shifter is used to convert one voltage into another voltage


• voltage regulator is used to produce constant output voltage from high voltage


• dc-dc converter is used to convert dc level into different level

Is this correct?

If I want to convert 5V voltage supply to 2.5V, do I need to use regulator, level shifter or dc-dc converter?