If your domain name expires, you will have a small grace period to pay for its renewal and keep it (at regular price). After the grace period which is usually less than 30days, the registry will require an additional redemption fee to reclaim it. In that period of time, your domain name may no longer function.
Once the redemption period elapses, the domain is usually made available to the public for new registration. Usually, it takes between 90 - 120days for the domain be available again for fresh registration, depending on the domain in question and the registry that controls the registration.
If your domain name is expired, there is NOTHING we can do to retain or reserve it again for you. That is beyond our control and is handled at the registry level. A registry is the central database organization that handles worldwide domain registrations. For example, top-level domains like .com, .net, .org etc are handled by ICANN (http://icann.org) while country code domains like .com.ng .net.ng and .ng etc are handled by NIRA (http://nira.net.ng)
Transferring an expired domain name is also not possible, it has to be active and within its registration time-span.
If you wish to keep a domain name, the best thing to do is renew it on or before expiry.