obvious questions

Dear list,

I read through the docs as well as I can, but I have some dumb
questions that I will need to be able to answer for users.

What makes a task different from an issue (other than the presence of
a severity column)?
I couldn't find task defined in the glossary.
I did find a section that suggested tasks were defined for actual
system administration of roundup itself?
https://www.roundup-tracker.org/docs/admin_guide.html?highlight=task#tasks

What is the purpose of the "Milestone" item?
Milestone is not defined in the glossary section of the docs, and a
search for "Milestone" in the documentation only shows the feature
list.
Should I define Milestones before I start making lists of issues or tasks?

-Dave