Am I a BWC or BES User?

BWC = BlackBerry Web Client
BES = BlackBerry Enterprise Server

BlackBerry Web Client
Choose the BWC data plan if the email account that you will be connecting to your BlackBerry is a POP3, IMAP or a Web mail type system. 
Some examples of supported accounts are:
United Online, NetZero, Juno Online, AT&T Broadband, AT&T WorldNet, BellSouth, Bluelight, Cablevision, Charter, Comcast, CompuServe, CoreComm, Voyager.Net, Covad, Cox, EarthLink, Qwest, RCN, SBC/Prodigy, Yahoo.

Web mail support varies by system.  Check with your service provider to verify that the account will support a copy/forward rule for incoming email or if it can be logged into remotely via the web.  Your BlackBerry handheld will integrate with up to 10 email accounts in total.

BlackBerry Enterprise Server
You will be using the BES service if your company has a BlackBerry Enterprise Server running in your email environment (check with your IT Admin).  Your company email must be a Microsoft Exchange or Lotus Domino system in order to support a BES server.  Also choose the BES data plan if you have Microsoft Exchange and you plan on using the Desktop Redirector software that comes with your handheld.  The BES data plan also includes BWC access so you can also connect up to 9 additional POP3 or Web Mail accounts to your handheld.