To newcomers, BES stands for BlackBerry Enterprise Server. Usually, this is of interest to businesses rather than individuals/prosumers.
Individual users who need BES Hosting or just want push email without the BES enhancements, you will want to see my BlackBerry FAQs
If you are a business who is new to BES: If your business is running Microsoft Exchange, and you need some solution that puts your corporate email onto corporate BlackBerries, you will want to read Blackberry Enterprise Server for Microsoft Exchange
. There's also a Lotus Domino
version of BES, as well as an upcoming Novell Groupwise