refresh / expire

[Reza Roboubi]

When we have a "refresh" rate, what is the purpose for an "expire" rate? 
Are the dns servers around the world not required to respect the refresh
rate??

Further, do refresh /expire mean:
        "lookup the _domain's_ own dns servers after this interval"
or do they mean:
        "lookup the root .com/.org/.net/... servers after this
interval"??

Thanks for your help.
-- 
Reza Roboubi
www.linisoft.com