easy to understand definitions

I found the following definitions but I need definitions that are easier
to understand.
I do not have my Bind book available.

# refresh - How often the slave servers should check the serial number
on the master for changes.
# retry - Amount of time a slave should wait before attempting another
"refresh" after a previous refresh has failed.
# expire - How long a slave should use it's DNS information without a
refresh from the master.
# minimum - How long a server should cache negative hits (e.g. no such
domain/host).

I don't understand "expire" and "refresh", it would seem that "expire"
should always be equal to the "refresh" time"?