Website Uploads Hi Def versus MP3

Ok, Im lost on this part.

I see on various members sites standard definition (which I assume is just a 44.1/16 bit MP3 File)

On others I see the option to listen to the Hi Def versions of a song.  Can someone explain exactly what the file is, bit rates, etc...  (Basically the entire Hi Def side)  I have no clue on exactly what it is.

Thanks in advance