ELI5: Docker containers

I can't seem to wrap my head around (Docker) containers and especially their maintenance.As I understand it, containers contain a stripped-down OS that shares some resources with the host?Or is it more like a closed-off part of the file system?

Anyway, when I have several containers running on a host system,Do I need to keep them all updated separately? If so, how?Or is it enough to update the host system, and not worry about the containers?