BBM has been an exclusive messaging platform on BlackBerry since it's launch. Now though, Blackberry have announced that it will be coming over to Android and iOS later this year.
Blackberry have planned for BBM to come over to the two rival OS' this summer and will offer text and photo messaging. Additionally at a later date, they will also include screen sharing, voice and video calls. All of these services will be free of charge.
It may be 5 years too late for Blackberry. BBM requires you have a PIN which you have to send to a friend, which then has to enter that in their BBM app. This is long-winded process compared to WhatsApp which simply requires you have a number hooked up to your account, if a friend has WhatsApp too, you can start chatting instantly. With WhatsApp claiming they having such a large active user base, which they're claiming is bigger than Twitter's, they own a massive chunk of the mobile instant messaging market.
BBM will have to compete with the likes of WhatsApp and iMessage which are a lot easier to use and setup. There are also a ton of other free messaging apps available for iOS and Android so I doubt BBM will have much impact at all. This may just be another desperate move by the company which is losing customers at a phenomenal rate.