When a domain expires, domain owners are given 40-day grace period to renew their domains. However, if the domain is not renewed, it will then enter the Redemption Period. This period will last for 30 days. During the Redemption Period, the domain owners can still renew the domain, but they are required to pay the $100 Redemption Renewal Fee plus the original domain renewal fee. If the Redemption Period is over and the domain is still not renewed, the domain will be listed as "Pending Delete" and it will later be released for general public registration

However, some people might find the $100 Redemption Fee is very expensive. Hence, it is very common that the domain owners decide to wait until the domain is released for general public registration. This approach is in fact very risky due to the following reasons

The Possibility of Losing Your Domain
All domains are registered on a first come first serve basis. Therefore, there is no guarantee that you can re-register your domain name after it is released for public registration. Also, a lot of internet firms often trace those easy-to-memorize domain names for re-sale purposes. Therefore, it is not easy to re-register an already-expired domain name after it is released for public registration.

Page Rank Issue
All domain names have a value of PageRank(PR). Newly signed up domain names have a zero value of PR. Assuming the PR for your domain is originally at 4. Once your domain is expired and has been released for public registration, your domain will now have a PR of 0 and you would need to put a lot of effort to re-gain the previous ranking.