What are ".aac" files and how do I play them or re-package them into a mainstream container such as m4a?

I have downloaded some audio files from youtube and they are in a format unknown to me which does not play in VLC or MediaMonkey. The extension is ".aac". I want to keep the files in the original AAC encoding, but I've never seen a ".aac" file before, only ".m4a", and I don't know what to do with ".aac" files.