A root text is a poem meant to be memorized and to be supplemented by a written or oral commentary. It is a "word commentary," one that paraphrases and expands the root text, which because of its brevity can be rather like a telegram.
All four schools of Tibetan Buddhism generally follow a similar curriculum, using the same Indian root texts and commentaries. The further Tibetan commentaries they use differ by school, although since the 19th century appearance of the widely renowned scholars Jamgon Kongtrul, Ju Mipham and Khenpo Shenga, Kagyupas and Nyingmapas use many of the same Tibetan commentaries as well. Different schools, however, place emphasis and concentrate attention on different areas.