"Certains" implies that there are "other ones". "Quelques" does not bring that precision".
For example, you have 10 tourists visiting a town, if 5 of them have lunch in a restaurant, you can say "certains sont allés au restaurant".
If you just say "quelques personnes seulement ont visité la cathédrale", it doesn't give any information about who they are, if they are a part or a group or not.
But the difference is quite subtle anyway !
both of it are undefined. but "certain" is more often a chick-in-tongue way to talk of "undefined" people that everybody have in mind.
I don't remember having heard of "certain" used talking of unliving matter. only people and concepts.
"certaines personnes pensent que vous mentez" = "some people thinks you are lying"
"certain l'aime chaud" = "some like it hot"
certains indices tentent à prouver que.... = some clues lead to establish that...
beware of the trap ! certain is also the word for "really sure"
"le fait est certain"
"vous nous suivez ? certainement !" = "do you come with us ? of course !"
"quelque" used for people is undefined to the point of becoming insulting. it suppose to mean "mundane, ordinary, common", but it really mean "unworthy of notice"