not the same, and no you don't need bbm installed.
A pin message is simply blackberry to blackberry via the blackberry pin ID's. It works in the same manner as BBM minus the personal info being traded.
A good example of why you would use pin messages is if you met someone here and wanted to talk further, you can give them your pin and that's it. With other message programs you would have to give out email address etc....
More info from google:
BlackBerry devices support direct PIN-to-PIN emailing; if you know the PIN of your recipient's BlackBerry, you can compose an email message using the PIN instead of the email address. This can be useful for troubleshooting, as a PIN-to-PIN message bypasses the email system entirely. Thus, when you cannot receive email normally but can receive PIN-to-PIN email, the problem is not with the device itself, the RIM (Research in Motion) network, or the wireless provider's network.