VB-DSN-ADO-DAO-RDO-ODBC

Dear All

I want to understand (specially difference of all above) 1) What is DSN, Why we make it?
2) Difference b/w these ADO/DAO/RDO/ODBC? Are these used for different database like ms access/oracle/sql server.
Please clarify. and pl. consider it urgent.
Thank you very much