A notebook is a blank book that you can write in. Students often carry notebooks, where they can take notes (and doodle).
The pages in a notebook are often lined, so that you can write neatly. You might keep one for writing to-do lists or thoughts (like a journal), or your notebooks might all be large binders full of class notes and math problems. Notebooks can be bound like books, or with a spiral metal or plastic binding, or consist of loose pages in a three-ring binder. Another, newer kind of notebook is a slim, lightweight laptop computer.