XI and BAPI

Hi all,

What is the difference between XI and BAPI, as I know one is middleware and other is interface, because using Business connector we can send the BAPI call as an XML to internet and viceversa can be done. Then why we are using XI for SAP to Internet (like EP) or viceversa. Please clarify in this regard.

and Is SAP System mandatory for XI? (for connecting two nonsap systems)