Smash books/journals are anything you wish them to be.
You can have themed pages, collections of images, lists, poems, songs, trinkets, souvenirs, hopes, wishes, dreams, pictures, etc. Like Pinterest in a haphazard free style book form. There is no rhyme or reason for anything in your book, if you like it, claim it on a page.
You can buy them and accessories or make them up yourself.
Quick Video Explanation