What is a junk journal?
A journal is a book in which you write, an art journal is one in which you paint and/or write. These journals are books made expressly for the purpose and are often made from really good papers so they can take wet art materials. A junk journal is made from junk paper that may be destined for the recycle bin or the trash. Sometimes they are based in an old book, altered for the purpose. A trashy junk journal is full of materials that mostly would not be considered 'good' paper, such as advertising materials (junk mail, flyers, catalogues).
What are they for, these junk journals?
You can write in them, paint in them, glue pictures in them, make lists. Some papers may need to be prepared with gesso or collage before art materials can be used on them.