Specifically, the original first season [iTunes link], is listed separately from the digitally enhanced episodes from the same season [iTunes link]. We'd like to say that the former is for purists, and the latter for the more adventurous among us, but iTunes customers have left comments stating that both collections feature enhanced episodes.
It's more confusing than The Menagerie.
Anyway, if you're a fan of TOS (and who isn't, really?), go and pick this up.