Dropped domain, you probably meant. A domain which has not been extended by the previous owner. Such domains are on sale, people buy them and put their own content in most of the cases.
Expired. If you buy/get it while it is expiring (ie before it is deleted and made available again) it does not count as drop but as a very late renewal.
Generally, in computer terminology, the term "Dropped' is often used as a synonym of "Deleted" or "Expired" or "Removed" or "Reduced".
drpped domains are domains that were dropped form registration. simply because the previous owner didnt wish/plan to renew it any longer.
Thank you for quick reply.I understood but how can I check the drop domain??Can you suggest any tool??
i think name.com offers facility for finding deleted domains. there are many other paid facilities too.
This thread can give you much information about dropped domains - http://forums.digitalpoint.com/showthread.php?t=513709