BlackBerry Enterprise Server (BES) lets a company connect its existing email system to a number of BlackBerry smartphones. It pushes email to the smartphones from behind a corporate firewall and allows the administrator to control what employees can access on their BlackBerry smartphones. Contact your IT department or BES administrator to learn more.
BlackBerry Internet Service (BIS) is for personal Web-based email and for small business users with POP3/IMAP email services. It's the easiest solution for those who don't work for a company with a BES server, or for those who want to use their smartphone for personal use.
You can use both BlackBerry Enterprise Server and BlackBerry Internet Service on your smartphone at the same time. This will let you receive your work email through BES and from other Web-based/POP3/IMAP email accounts (e.g., Hotmail, Bell Internet) through BIS.
If you have Web access to your work email (MS Office Outlook Web Access) you may also be able to set your BIS account to receive your work email that way. Learn more at BlackBerry.com.