But what *is* a {system, user} package?

I am no Linux expert, but the basic premise seems simple and explained elsewhere: A system package is installed sytem-wide, in particular they affect all users and environment variables. A user package can be configured on a per-user basis (and available/ exposed to only a specific user?) and so on. What I still do not understand is: What should be considered a system or user package? I am the only one using my laptop, but I have to wonder if there is still a distinction. In any case, I ...