An expired domain is one that the owner has failed to renew. This can be for several reasons. Prehaps they forgot to renew it, prehaps they went out of business and forget about domain, or they simply don’t care about it anymore.
After a domain expires and is not renewed within 30 days, it will go up for auction. Bidding will last for 7 days, and after that, the domain will be given to higher bidder, and with the high bids you can buy expire domain very easily.
And with this process, you can grab many good expiring domain.