There is a lot of info on BES if you search. And our FAQ has a section on it as well:
BlackBerry FAQ - BlackBerryFAQ
BES = Blackberry Enterprise Server
BlackBerry - BlackBerry Enterprise Server | Integrate Enterprise Applications, Data & Wireless Communication
You would not have it on your device. It is generally used in the corporate environment. Single users can use a hosted service. Check out exchangemymail, one of out corporate sponsors.
ExchangeMyMail - Hosted Exchange 2007 Mailbox and Blackberry Synchronization