Difference between SAP BAPI and BAdi and Customer Enhancement vs Customer Modification?

BAPI

It is a short form for Business Application Programming Interface. It is a simple API used for business operations. It can be simple module function provided by SAP or it can be developed by the user as well.

BAdI

This is an abbreviation for Business Add-Ins. These are customer exits which are used to modify or customize existing SAP functionality. BAdI can be reused as they follow Object-oriented approach.

Enhancements

It is a modification of SAP functionality or adding new functionality possibly by using a BAdI class or any enhancement technique.

Customer modification

It is to customize existing function modules as per customer requirements.