I'm interested in a domain-name (.com) that is about to expire. What happens when the registration-expiration date is reached?
After the expiration date is reached, usually the registrar (the entity that takes care of registering that domain and who associates it with you) keeps the domain on hold for 1~2 months (can be more or less), having you (the current owner) as the priority-subscriber/renewer during that time.
In other terms, the domain is not yours anymore (for instance the top-level-domains DNS delegations are not directed to your DNS entries) but only you may register/renew it.
After that grace period is over, registrars follow one of the following scenarios