You are correct. Here are definitions of both services and other services.
Stands for BlackBerry Internet Service. This is your @blackberry.net email account, which you can access via a web browser, in order to configure your BlackBerry through a web-based interface. You can also read your blackberry.net emails on your BIS account. This is the new name for the BlackBerry Web Client (BWC).
This stands for BlackBerry Enterprise Server. BES turns a BlackBerry into a powerful government-quality secure email system with full wireless email and PIM synchronization, including email folder structure, moves between foldres, deletitions, unread/read indicators, etc. An administrator can even kill a BlackBerry remotely through BES when it gets stolen too. People who need inexpensive BES, can see the Hosted BES FAQ. BES also provides MDS capability, which is generally higher-reliability than the recently available TCP/IP stack found on BlackBerries.
Here is a FAQ about Hosted BES The Hosted BES FAQ - Cheap BES/MDS - Wireless Outlook sync!
MDS is a component of the BES
This stands for Mobile Data Service. It's a method of Internet connectivity for a BlackBerry. that is provided by a BlackBerry Enterprise Server. In the past, BlackBerries required MDS to run most kinds of third-party Internet software. More information can be found at Configuring Internet on BlackBerry. BlackBerry Internet - MSN/chat/web/telnet/TCPIP - No BES!