A sketchbook is a book or pad with blank pages for sketching and is frequently used by artists for drawing or painting as a part of their creative process.
There are, perhaps, two broad categories for classifying sketches:
Observation: this focuses on the documentation of the external world and includes many such travel and nature studies.
Invention: this follows the artists' digressions and internal journeys as they develop compositional ideas
Sketchbooks come in a wide variety of shapes and sizes, with varied covers, and differing numbers of pages. Sketchbooks began as a way to provide a readily available supply of drawing paper in the convenient form of a book. Finish of the work found in the sketchbook varies widely from artist to artist, with some having very simple drawings and lots of notes, and some having highly worked images. Over time, it might allow others to see the artist's progress, as their style and skills develop. Many artists personalize their sketchbook by decorating the covers. Sketches are sometimes removed from sketchbooks at a later date.