Hi,
BIS stands for BlackBerry Internet Service and it is a services that is designed for users who don't have their own mail server or are unable to install BES. This service gives users the ability to wirelessly send and receive emails from services such as POP, IMAP and OWA. Unlike BES users have to manually synchronize calendar, contacts etc via USB or a Bluetooth connection to their PC. If you are a BES user you also have the ability to add BIS email accounts which allows you to also access personal email on your device (The BES administrator has the ability to block this service). Here is some further information for you:
BlackBerry Internet Service at BlackBerry.com
Thanks,
Gary