What happens if I fail to renew a domain name?

Domain > Registration
When a domain name expires, it will go into Renewal Grace period (which is 30 days) whereby only the current owner of the domain name can renew it.
After 30 days from the expiry date, it will go into a Redemption period, which is another 30 days.  During this period, the owner of the domain name will be able to renew it by paying the renewal fee PLUS a redemption fee, set by the Registrar.
In summary, the domain name will be released to the public for re-registration about 60 days after expiry and when it is "not found" when you do a WHOIS search.