I'd say that a thriller tends to have a plan, which is slightly different from horror - in true horror, the existence of the source of horror fuels the book - as in Misery or that car thing - there're things that can be found out, but very little in the way that can be solved - unless we mean the wholesale destruction of said horrific thing.

Thrillers tend to have their plots fuelled by the plan of some person, thing, organisation - I think of thrillers as more organised sort of plots in general. And thus, when stopping the bad thing that is happening in the thriller, I tend to think there's more mopping up to do (whether we see it or not) after the bad guy/person/institution/thing is stopped. If you survive the horror, you walk away, end of story.

Which is very vague of me, and King himself has loads of books that overlap between both definitions, except they're not always as scary. Senseless things are always more frightening, since we can rationalise them. <is huddled in blanket>