Martial arts are codified systems of both armed and weaponless, or empty-handed, skills intended to develop and increase proficiency in combat.
Martial arts have arisen in nearly all cultures, both through cross-cultural transmission and independently. There is some form of codified martial art, either historic or current, identifiable in virtually every culture around the world today.
Since every culture has its own unique value system, code of personal honor, cultural duties and responsibilities, religion(s), politics, geography, raw materials, climate, etc., almost no two systems are exactly alike.
This has rendered the scope of martial arts so enormous that it defies a complete, concise, logical, universally accepted categorization.
Another broad category (for our present age) could be "Lethal" vs. "Sport Martial Arts." Even these categories are clouded by the fact that several Martial Arts which were lethal earlier in their history and no longer taught as such due to the removal of certain techniques and ways of thinking. Indeed, in some cases, these lethal aspects have been lost forever and not merely hidden.