A textbook is an informational book that you use specifically for a class at school. Your biology textbook might be so heavy that you dread lugging it to class.
Textbooks are manuals written for instruction in a subject, rather than books you might read for fun. Today many students read textbooks in electronic form, rather than as bound hardcovers. Starting around the 19th century (with easy access to printing and standardized education in many parts of the world), textbooks became a common educational tool. Earlier, a textbook was a name for "a book with wide spaces between the lines."