|Flickr image from Super Furry Librarian|
Archivists have a different point of view and orientation to the retention and organization of materials. Librarians are all about access--making sure people find things they want, organized to that end. Archivists are more focused on the material, keeping collections intact. So if Bob Smith's family donated Bob's letters, diaries and scapebooks on Bob's hobbies, it is important to an archive to keep all of Bob's stuff together. If Bob's stuff went to a public library, more than likely, it would all be split up according to topic and arranged by subject.
It's like there are cat people and there are dog people. They both like four legged furry friends, but then things split.