protected aac is .m4p as appose to .m4a for unprotected AAC. Protected AAC is only able to be used on the iPod, and can only be used on 5 computers you "authorize." Also, they can not be used in iPhoto slideshows, and I'm not sure about DVDs.
There is many different software titles that remove the protection, the best being HYMN. It converts .m4p to .m4a without data/sound loss.