BES stands for Blackberry Enterprise Server and represents the BlackBerry email service provided by a server ran by a company
BIS stands for BlackBerry Internet Service and is the BlackBerry email service from your carrier. your carrier then allows you to access your own emails.
So: If you bought your BlackBerry in shop, you probably have BIS, if you got it from your company at you can also synchronize your addressbook or calendar you probably have a BES