Despite iPhoto's long history, many people continue to be confused about exactly what happens when you delete a photo. There are three possibilities.
If you delete a photo from an album, book, card, calendar, or saved slideshow, the photo is merely removed from that item and remains generally available in your iPhoto library.
If, however, you delete a photo while in Events or Photos view, that act moves the photo to iPhoto's Trash. It's still available, but...
If you then empty iPhoto's Trash, all photos in it will be deleted from the iPhoto library and from your hard disk.
From Adam C. Engst