put it in the trash:
the item has been moved from its original location. BUT an invisible hard UNIX link remains. you cannot ever see this (even in terminal). the trash is a logicalk location for items you want to delete.
empty trash:
the item is destroyed. the HDD space it took up is available again. the link is also destroyed
PS. if you delete the trash folder, you delete the item. when you next either star-up or run a disk operation (i.e. index, or fsck -y) the link will be destroyed. it is also destroyed if something trys to access the file when it does not exist.
hope it helps