When a domain name hits its expiration date you have approx. 60 days [exact number depends on the orginial registar] until you can purchase the domain name. If you check the whois on a domain name [my favorite to check JUST the domain status and expiration is whois.domainpeople.com] you will see one of the following statuses, listed below in the order of occurrence:

Active - Will show from approx. 30 days after registration to 5 days after expiration.

Registar-Hold
Redepention Period
Pending Delete - Approx. 5-7 days after pending delete starts, the domain name will be available, except domain name backordering systems can get you the domain name after 2 days but you won't see your whois ownership until after the 5-7 days.
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Backordering is much better then attempting to get the domain name yourself. Because if someone else is back ordering it you have a better chance of getting the domain name compared to if you don't use the back ordering services.