That is not really a question in the focus of WordPress, is also in other programming languages and in our speaking language a topic. Maybe I can clear the difference between these words with a non coding example.
A return of a situation to a state. Like a call with the telephone to a message of the phone or the answering machine to a defect of a product.
In the coding context is a callback a function/method which executed after the current effect is finished.
An act of falling back. Like an alternative plan to the current idea, progress.
In context of the computer is a fallback typically a backup of the current data to fallback to a time stamp.