Please tell me about Enhancement Technique like BTE , SET and Formula

Hi Experts .

Please guide me about Enhancement Techniques used in SAP apart from BADI/User-Exit .

1.) what is set and how to create it ?

2.) What is formula is SAP and How to set it for Enhancement Pupose .

3.) What is BTE (Buisness Transection Events) ? How to create it for Enhancement Pupose ?

Please Elabarate the reply with step by step procedure .

Thanx .

Regards : Rajneesh