Droped domain name is non expired domain name, but the current domain name owner id not longer intersted in this domain name.
An expired domain is one that a registrant either forgot to renew or has decided to let drop. After a names registration period expires it then goes into redemption for 43-90 days and if the old registrant doesn't pay the fee to have it renewed/re-activated the name is then dropped and available to register.
I found a great image that depicts the life cycle of a domain name and posted it to it's own thread so that everyone can find it. Check it out: http://forums.digitalpoint.com/showthread.php?t=1719250 Hope this helps you
dropped domain means the one which has removed out expired domain means the one which has finished its interval or validity