Secondary DNS replication

Hi..

Im hosting a primary DNS server for 200 domains at my office. The secondary is off site. In the properties page of the SOA for a domain in the DNS mmc there are 3 fields, namely, Refresh interval – retry interval and expires after. Can anyone shed some light on exacly what these fields refere to? You can also set the TTL for hte SOA, but what do the numbers refere to. Is it day, hour minuite seconds, or what? All these fields direcly influence DNS replication and the amount of time your secondary will stay up in the event of a primary failiure – so – can anyone help?