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
A Circle Journal is a book passed around to a group to add too by other members of the group, like a round robin swap. Some are themed some are not allowing members to be creative.
So each member has a book (reasonable size, easy to mail), and they send it to the next person on the list below them, who adds to it (time limit to create) so forth and so on and eventually after a set amount of time, you get your journal back with all creative pursuits on a page or two from the people in your group.