zone file settings question

[Madison Kelly]

Hi all,

   I was trying to find out what certain settings effect. For example, 
in a zone file with the following...

-=-=-=-=-
$ORIGIN .
$TTL 38400	; 10 hours 40 minutes
domain.com		IN SOA	ns1.nsserver.com. admin.domain.com. (
				2007101501 ; serial
				10800      ; refresh (3 hours)
				3600       ; retry (1 hour)
				86400      ; expire (1 day)
				38400      ; minimum (10 hours 40 minutes)
				)
...
-=-=-=-=-

   What do these fields mean?

- refresh
Would I be right in guessing that this time is how often a client would 
try to check the zone for updates, but failing to get a response would 
keep using the TTL so long as it was valid?

- retry
Is this the amount of time, after a failed attempt to contact the 
server, that a client would wait before trying again?

- expire
How is this different from 'ttl'?

- minimum
Not sure even of a guess...

   I hope these aren't "obvious" questions... I tried reading the docs 
at the ICS website, but couldn't find these details.

Thanks all!

Madi