When a domain expires (when it reaches the expiry date mentioned in the public WHOIS database), It will be held for a grace period which can be anything from 1-45 days. The actual period is decided by your registrar.
During the grace period all associated services will cease working until the name is renewed (if that happens). Nameserver/DNS changes will not be possible as well. If a renewal is not made by the end of the grace period the domain will sent to the registry for deletion and returned to the available pool. Deletion at the registry level can take 1 to 10 days.
However the .com, .net and .org domains will be held at VeriSign (the .com & .net registry) for an additional 30 days after the end of the grace period. This period is called "Redemption Period", and it is still possible to recover the domain name and have it renewed.
Hope that answers your question.