BlackBerry Enterprise Server (BES)

BlackBerry Enterprise Server (BES) is the name of the middleware software package that is part of the BlackBerry wireless platform from Research In Motion.
BES connects to messaging and collaboration software (Microsoft Exchange, Lotus Domino, Novell GroupWise) on enterprise networks to redirect emails and synchronize PIM information between desktop and mobile software. Some 3rd party alternatives for the above 3 mail platforms exist ( Scalix, Zarafa, Zimbra), but are not supported by RIM.

BlackBerry Internet Service (BIS)

The BlackBerry Internet Service, or BIS, allows BlackBerry users to access the Internet and POP3/IMAP/Outlook Web Access email accounts without connecting through a BlackBerry Enterprise Server (BES). The service is usually provisioned through your mobile phone service provider, though Research In Motion (RIM) (BlackBerry’s developer) actually runs the service. This service uses RIM’s proprietary push technology which allows real-time access of email.