FLAC is a perfect example of a true lossless codec. Think of it like a ZIP file on your computer, what comes out of a ZIP file is exactly the same as what went in.
I haven't looked into the HD (DTS or Dolby) codecs but one thing that raised an eyebrow was Dobly's claim of "up to" 4:1 compression. To my knowledge no data packing algorithm can achive 4:1, only psycho-acoustic codecs that compress audio can ... this may be why they state "up to". Now they might be deploying other methods liking dropping frames/data when no audio data is present on some channels but that's just a guess.
