is there a concrete difference between thriller and horror?
I think it depends on the plot and characters of the movie. You can take a movie like The Silence Of The Lambs that has serial killers and mild gore scenes and one could think of it as a horror movie. It has horror elements and a lot of people probably think of it as a horror movie. But you could qualify it as both genres.
But a movie like Double Jeopardy is very much a crime movie as well as a thriller. I do not remember it having very many horror elements which is why it is probably mostly listed as a thriller. But something along the lines of Along Came A Spider might fit into both genres, depending on your perspective.
I do not really get into the whole genre labeling and sub genre thing.