The app was previously only available on the iPhone. The iPhone version could be installed on the iPad but you had to blow it up to fit the screen size, and that's which is never really a nice experience.
There is nothing particularly new about Messenger other than it has been redesigned for the iPad interface; the UI actually looks a little bit like the UI you see on the standard Apple Messages app.
You have all your open conversations appear in a list down the left-hand side and then you can simply tap between them to interact and type messages.
In April Facebook revealed it was killing off in-app chat on the iPhone Facebook app, so now any Facebook messages you now get can only be access in the dedicated Messenger app. Although this can be accessed in the main Facebook app by pressing on the Messenger tab, which will then automatically switch applications.
It hasn't been confirmed whether or not Facebook plans to do the same with the iPad Facebook app. Chatting to friends on Facebook for iPad isn't really an issue considering the amount of screen real estate you have.
To download Facebook Messenger for iPad hit this link.