[fourstringfuror]

BBM stands for BlackBerry Messenger, and is just one of several BlackBerry features.

A BlackBerry data plan will give you the core BlackBerry functionality - email, multitasking, BBM, browser, etc. BBM Voice is part of BBM and works over Wifi, so you must be on Wifi and calling another BBM user (also on Wifi).

Without a BlackBerry data plan, you have very limited capability. You can make calls and send text messages, but little else, in most cases.