What happens to expired domain names?
A variety of things can happen to a domain once it expires. It can be renewed by the registrant, be purchased at auction, return to the registry (the primary database for domain names), or become subject to domain squatting. Before any of those things happen, the party who was leasing the domain (the registrant) will usually have opportunities to buy back the rights to use their expired domain name. However, it's much simpler to renew the domain name before it expires.
How does a registrant know when their domain is about to expire?
It is the responsibility of the registrar to alert its customers when their domains are about to expire. However, some predatory registrars will instead wait until the domain expires, buy the domain, and then attempt to sell it back to the original registrant at a much higher price. This is a form of domain squatting.*
Registrants can also check the expiry date of their domains themselves via a WHOIS lookup. WHOIS data includes who owns domain, registrant contact details, when the domain was purchased, and when it expires. WHOIS is not a centralized database; rather, the data is maintained by the various registrars and registries that are certified by ICANN (Internet Corporation for Assigned Names and Numbers).
*Domain squatting, or cybersquatting, refers to registrants who register domain names before other people or organizations that want them have a chance to do so, often in violation of trademarks. Cybersquatters sometimes will attempt to extort money from those who should rightfully own the domain. Domain squatting is different from domain parking, which is when someone registers a domain they are not actively using without attempting to sell it to someone else.
Can the original registrant renew expired domains?
Typically there is a grace period during which the registrant can still buy the domain back without paying additional fees, but the length of it varies from registrar to registrar. It could be as short as a week or two, as long as a year, or not offered at all (although reputable registrars offer a grace period). The length of the grace period also depends on the kind of top-level domain (.com, .org, .net, etc.) used.
Following the grace period there is usually a stretch of time called a redemption period, during which the registrant can still renew their domain. Registrars typically require a redemption fee for renewal during this period in addition to the renewal fee. The redemption period is only a few weeks long, 30 days at the most.
When and how can a new registrant buy expired domains?
The grace period and the redemption period (if the registrar offers both) have to expire before someone who is not the original registrant can buy the rights to use an expired domain name. These two periods together could add up to only a few weeks, or could last well over a year. Following this, the domain is in a 'pending delete' stage for a short stretch of time.
After these periods expire, the registrar may auction off the domain on the open market. At this point the domain can be purchased by the highest bidder. However, not all expired domains are put up for auction.
What does pending delete mean?
A domain is in a state called 'pending delete' for approximately 5 days once the redemption period ends. Pending delete means the domain is about to expire, but it is not yet available to the public. The domain is not actually deleted when the pending delete period ends, but the WHOIS record is.
Can a registrant delete their own domain?
Domains aren't deleted in the sense that they cease to exist permanently, but some registrars allow registrants to terminate registration of their domain. Other registrars do not allow registrants to end their registration until the domain expires naturally. In both cases, the domain will usually be made available to the highest bidder afterwards.
What happens if an expired domain is not purchased?
If no one purchases an expired domain, it may be returned to the original registry after a certain amount of time. It will not be available again until the registry decides to release it.