what is the difference if i choose my database through ODBC and through active data(ADO) in crystal report?

i use ODBC and there is message "server has not yet been opened" appear when i run my application.
so that i change using ADO.and it works.
Why it doesn't work if i use ODBC? How do i open the server?
i use vb6 and ms access2002
many thanks for your answers