Check out Link2Exchange
or Exchange My Mail
for hosted solutions for your Blackberry. They're very easy to set up, much like adding an email account on your Blackberry. You pay an additional monthly fee for these services, on top of whatever you currently spend on a Blackberry Data Plan now.
BES is Blackberry Enterprise Server, the corporate data connection where your company connects you to their internal Blackberry mail system. The above links are similar solutions but for individuals who don't have a big corporation running a BES.
BIS is Blackberry Internet Service, the data plan type that most of us non-corporate types use. What you lose with BIS vs BES is wireless sync of your calendar, contacts, and full email sync. Lately, however, lots of email providers have upgraded their services so if you set up your personal email account as an IMAP account and use BIS, you get some of the email syncing previously only for BES users. For example, my Apple .Mac/MobileMe became instant push to my BB recently. I also get my sent items automatically put into a "sent items" folder on my home computer. And if I open an email on my BB, it shows up as read on my desktop. Similarly, if I delete an email on my BB, that email is in a "deleted items" folder on my desktop. But these features are on a provider-by-provider basis and can be inconsistent.