I've seen the movies countless times, and in total have seen almost EVERY RGB episode. But I guess when you're a kid, you don't catch onto these things…or maybe they just didn't explain it to begin with. How do you seperate the classes? IE What's a class 4? Class 6? What classifies them. A post I read mentioned that if you were once human you're a class 4.. so im assuming other classes are defined to according to whatever logic this person posted under. As big of a GB geek as I am, I don't know this one. Any clue folks?

Sparks

I think that they're separated by what the ghost was before it was a ghost (if it even was something before that) and by PKE. But that's just my best guess.

Here's a link to what is supposed to be a description of the classes: http://hometown.aol.com/janmelnitz/terminology.html
However, it appears to be incomplete, as it only goes up to 7, and there are 10 classes. Here's my understanding of the class system:
1. Undefined forms, ectoplasmic vapors, etc.
2. Defined manifestations, but not very powerful.
3. A ghost that was once human, but the identity of which is not known.
4. A ghost of a human whose identity is known. Not sure why the distinction, but I suppose it makes a difference when trying to decide how to deal with it.
5. Non-humanoid manifestations, which supposedly (in the GB universe anyway) the result of various rituals, catastrophic or emotional events, etc.
6. Ghosts of living creatures other than humans.
7. Demons, usually rather powerful, but with enough energy, can be captured conventionally.
8. Elementals, powerful spirits that are identified with the “elements” of the ancient world: wind, fire, water, and earth. These are very powerful and extremely difficult to trap conventionally, so usually they must be dealt with by researching the ancient customs surrounding them. If anyone remembers “The Collect Call of Cathulu,” I believe Cathulu was an elemental.
9. For the life of me, I cannot remember what type of ghost inhabits this class. Anyone out there remember any references to this?
10. Demi-gods, e.g. Gozer, Samhain (I believe he is one, but I could be mistaken; anyone have an episode with him in it who can tell us?), etc. Highly powerful. Though it could be possible to capture them, normally the best way to deal with these nasties is to send ‘em back where they came from. I suppose Vigo fits into this defintion as well, although he was human, but I suppose they could bend the rules enough to let him in.
I know that it’s just a comedy, but I'm the type of person that picks up on the science of such movies. I guess I'm just a bit of a geek that way, though in truth I think I like Ghostbusters so much because it combines comedy with science fiction. Of course, I'm sure I've made some mistakes in there. Anyone else have additions to that?

I know what you mean about picking up on the science in the movies. I thought they mentioned a class 12 in one of the RGB episodes. I'm probably wrong on that one though. Nice list of the classes though.

They did mention a 12 and I disagree with that class system.

As a recall from the EGB episodes “Back in the Saddle Part 1” and “Back in the Saddle Part 2”, anything above a Class 4 was rare in the old RGB days.

My best guess is, since Ghostbusters was a cartoon and the writers probably didn't expect most kids to pay too much attention to most of the more minor scientific stuff (ha! fools), they probably didn't adhere to strongly to some of that stuff. I remember hearing parts of episodes where they'd mention something like a class 12, and another would say, “But there's no such thing as a class 12!” so I really don't think there was a huge push for accuracy. Just look at “Back in the Saddle;” they called those little ghosts that could reproduce class 4s, when by that classification system, they should have been 5s.
You think this is a debatable topic? Check out Trekkies when the topic of warp speed enumeration comes up.

Strange indeed, I remember in the RGB episode “Ghosts R Us” that the ghost scam artists woke up a class 10 and they caught it simply because slimer made it fall over, then they just trapped it :/

Meh?

I agree with Bluestancontra or whatever his name is…but hes right.

I swear Egon said it well over 10, I gotta watch the tape again and check.