Business Add-Ins (BAdIs) 
Definition
SAP Business Add-Ins (BAdIs) are one of the most important technologies used to adapt SAP software to specific requirements. BAdIs were introduced with Release 4.6 and replace function module exits. This technology is not limited to SAP applications. BAdI calls can be integrated in customer applications. These can then be enhanced by other customer applications. In the various SAP applications, BAdI calls are implemented at places where enhancements are appropriate. In the SAP ERP system, there were already approximately 5,000 BAdIs before Release 7.0.
Use
As of Release 7.0 of the SAP NetWeaver Application Server ABAP (SAP NetWeaver 2004s), BAdIs are part of the Enhancement Framework, where they represent explicit enhancement options.
We
call these BAdIs 
classic BAdIs. In
addition to their integration in the Enhancement Framework, BAdIs from Release
7.0 onwards differ from classic BAdIs particularly with regard to the fact
that they are supported directly by the ABAP runtime environment through ABAP
statements.
For more information about BAdIs, see:
BAdIs - Migration of Classic BAdIs