I found this in Mark's Blackberry Glossary:
"This is an acronym for Access Point Name. This is used for the TCP/IP stack. This is a kind of a gateway between the mobile network and the Internet network. To gain access to always-on Internet from a mobile phone, such as instant messaging, it goes through an APN. Some mobile phones, such as BlackBerry uses multiple APN's. The main APN is blackberry.net for the BlackBerry emails, but there are other APN's that are carrier-specific for getting Internet access such as Verichat, etc."
See this thread for the APN's for different carriers... BlackBerry Internet - MSN/chat/web/telnet/TCPIP - No BES!