"With BBM for iPhone users are able to use the Blackberry Messenger application on their iPhone’s, to chat with Blackberry devices all arround the world. The app is download for more then a thousand times already so it is very popular."
BBM is BlackBerry's version of iMessage, but it's strictly (until now) for BlackBerry-to-BlackBerry messaging. BlackBerrys all have unique PINs (Personal Identification Numbers) hard coded into them at the factory. This 8-character PIN is the basis for the BBM app. The neat part is that BBM messages go out and come in over your data connection, not as text messages. Lots of people still pay by the message for texts. BBM avoids this, is very quick, allows for delivery confirms and read receipts, is very efficient (typical BBM messages are measured in kb), and is very secure. Now it's gone cross-platform. You get assigned a PIN from BlackBerry when your reservation comes up. Mine took less than 24 hours after downloading the app.
The big difference with iMessage is that BBM never had any provisions for sending messages to non-BlackBerry users until now. It was strictly for BlackBerry-to-BlackBerry communications. iMessage will at least send a message as a regular text message of the recipient doesn't have an iPhone. This cross-platform release of BBM for iOS/Android finally opens it up, but only if you have the app installed.