KillerSpots, DeSpot, SpotLess

[Miranda]

I'm trying to figure out how these filters differ, and what exactly each filter does differently. Any info would be welcome

[Selur]

They all aim for the same: Spot/dirt removal.
DeSpot, is probably the most aggressive of them, then SpotLess and then KillerSpots.
Since sadly, RemoveDirt is partially broken in Vapoursynth only the basic part of KillerSpots can be used.
Personally I mainly use DeSpot and KillerSpot and rarely SpotLess.
In general I would recommend to try all three depending on the source one or the other might be more suited.
(for cartoons, KillerSpots usually is better)

Cu Selur

[Miranda]

What do these filters actually do? Do they remove grain, or larget dots?

[Selur]

Usually only larger (dynamic) grain might get removed, but best simply test it out.
There are some folks over at doom9 who are looking for ways to get more control over what gets removed when those functions are used in Avisynth, but to my knowledge none of these have been ported to Vapoursynth.
Side note: Sometimes it can help to restrict those functions through a mask. (Hybrid has some basic masking support, which probably will be extended in upcoming versions. Dev version already can to a bit more than the last release. To enable the masking controls enable Filtering->Vapoursynth->Misc->UI->Show 'Masked'-controls)

Cu Selur