Remathilis
Legend
Strangely enough, it makes sense how Pathfinder define the stealth concepts. This one is just codifying what (basically) existed in 3E before it.
Unnoticed - you don't even know the monster is there. You're flat-footed against it, and some abilities it has work against you (assassination, I guess).
Undetected - you know the monster is there, but not which square it's in. You have to choose the right square to attack, and there's a 50% miss chance, and you're flat-footed against it.
Hidden - you know the monster is there and where it's standing, but you can't see it. There's a 50% miss chance, and you're flat-footed against it.
Observed - you can see the monster. Normal rules!
Meanwhile there's
Invisible - You're undetected until someone notices you (with Seek), then you become Hidden.
Concealed - You can't see the monster clearly. 20% miss chance.
Because Pathfinder like pinning down stuff, you also have Hostile, Unfriendly, Indifferent, Friendly and Helpful for NPC attitudes.
Cheers!
It seems like a lot of keywording for keywording's sake. Unnoticed, Invisible and Undetected are basically variants of Hidden with a minor adjustment to exactly how you try to find them. In specific, there is no difference in Undetected and Hidden except the guesswork of choosing a square to attack (either you have an educated guess and they are Hidden or you don't and they're Undetected). It appears a correct guess on an Undetected foe just makes them Hidden, and a fireball where you think they are ignores the difference completely. That is some fine-hairs to split, especially for something so corner-case that they 90% of the time play out the same.
More importantly, I suspect there will be plenty of Unnoticed/Undetected mixups due to their similar names and very minor difference in effect; and I totally suspect some would-be assassin is going to think he gets death attack because his cloak of hiding gives them the undetected status (while death attack requires the unnoticed status) as an example. Hell, I played 3x for 10+ years and I can't tell you the mechanical differences in shaken, frightened, and panicked without the SRD open.
I just feel that level of precision is maybe better suited to some sort of computerized play (either in the form of an hyperlinked SRD, a virtual TT, or a video game) because I couldn't fathom trying to keep those states straight using just my memory and a hardback book...