It's enough to make a grown adult cry. I don't know the specific taxonomic differences that are used to classify them, but the layman's version is ...
Caudatum - The big bad wolf of the group. Flowers can be fairly dark, though usually much less than a typical P. humboldtii, to green/white/pale. Personally I've never seen one as small and compact as the one in this thread, but I'm not going to pretend I'm knowledgeable enough about the variation in this species to weigh in on the ID.
Humboldtii - Dark flowers, formerly classified as warscewiczianum. Small plants. Flowers are often fragrant (I had to throw that in there).
Warscewiczianum - Pale flowers, formerly classified as wallisii. Small plants.
Wallisii - Outdated synonym for warscewiczianum.
Lindenii - Pale flowers, peloric with 3 petals instead of a slipper lip. Small plants. Very similar to warscewiczianum, other than the peloria.
Exstaminodium - Dark flowers, small plant, missing staminode. Very similar to humboldtii, other than the missing staminode.
I think there may be at least one newly discovered species that's been placed into what I colloquially call the "Caudatum group", but at the moment I don't have my resources with me.