When one do not pay for the domain name anymore before the expiration date, the domain get expired. Here are some facts about the expired domains.
Buying expired domain and parking it with the intention of selling at a higher price is a well known business now and also a good expired domain can be used for own usage; but in the later case, the previous karma of the domain name must be checked : if it was a black listed domain by Google.
In reaching its expiration date, a domain name can go to following condition:
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Reminder emails or phone calls from the provider: Not all Registrar gives this option. Check at regular interval how many months are left. It is better to pay beforehand for the next 12 months (at least).
Grace Period: The period given by the Registrar for the domain owner who pays the fee to renew the domain. The number of days of this period can be from 0 to 60 days, varies by the Registrar which should include this in their Terms of Service. The renewal fee should be the same.
During this period the domain may still remain functional, although it is likely to display a pending renewal notice.
Redemption period: If the domain owner did not pay the renewal fee for the previous period, he / she will get 30 days to do so. The domain is no longer in the DNS zone and so will not work and it is likely that the Registrar will ask for a higher fee (10 to 20 times more) to get back the domain working.
Pending period: After 30 days of the redemption period, the domain is put on hold to be removed from the Register. At this point, the owner can not recover the domain, the domain is on hold for 5 days before being available for registration again to another (or the same) domain owner. The exact time at which the domain is free is probably depends on the Registrar. The public WHOIS information is updated too.
Tips for top level domain owners
- Keep it renewed before reaching the complicated periods.
- If you are interested to backorder any domain name; suppose your company’s name is NAME and Top Level Domain is going to be expired, you can search for various auction services to backorder.
- Read all the FAQs, Policy of the Registrar before buying a domain name; if you find anything disagreeable or anything else, you can transfer the domain name after paying a fee.

It is quite true Abhishek about domain names that have expired. Unfortuantely I have purchased many that have been blacklisted by Google, and it is very hard work to reverse, as you have to prove you are a new owner etc.
Always check before you purchase a domain.
Thanks.
Norbert
Personally contact me. Lets see what we can do for those.