iPhoto Librarian can be used to launch iPhoto with a different library folder. This allows you to have multiple collections of photos that are not viewable as one large collection. The advantage of this is if you have one or more collections of photos which you don't want mixed together.
By default iPhoto's library folder is located under "iPhoto Library" within the Pictures folder of your Home folder. iPhoto Librarian allows you to maintain multiple such folders of images.