What is the difference between 王 and 皇?
The generic way to refer to the highest ruler of any country/region is「君主」, corresponding to monarch.

If we talk about an English translation, specifically avoiding how these terms are used in China, Japan, Korea, and Vietnam, then「國王」is used to translate the English equivalent of king and「皇帝」is used to translate emperor, which are strictly gender neutral. Their specifically female equivalents are「女王」and「女皇」, respectively, for regnants (non-consorts), and「王后」and「皇后」for consorts.


Margrethe II,「丹麥女王」 (Queen Regnant of Denmark)
Augusta Victoria of Schleswig-Holstein,「德國皇后」(Empress Consort of the German Empire)
Napoléon Bonaparte,「法國皇帝」(Emperor of the French)
Salman bin Abdulaziz Al Saud,「沙特阿拉伯國王」(King of Saudi Arabia)



  Equivalent titles for territories historically surrounding and interacting with China, like Khagan and Raja, have their own Chinese transcriptions (可汗, 羅闍).




In Chinese-language influenced regions, the situation is different, and whether a person holds the titles「王」or「皇」is more complicated.


「皇」as a monarchial title was invented by Qin Shihuang, "the First Emperor of China". The「君主」of the previous Zhou Dynasty (and earlier) held the title「王」. Note that the Zhou Dynasty themselves invented another term for highest ruler, called「天子」(son of heaven), which wasn't shared by the predecessing Shang Dynasty. The timeline for the "highest ruler of China" title is something like:


Shang Dynasty:「后」>「王」(Yes,「后」was a title originally used by Shang Kings. The meaning queen (consort) arose later.)
Zhou Dynasty:「王」,「天子」
Qin Dynasty and beyond:「皇」,「天子」
Occasionally for non-Han rulers of China, in addition to their Chinese titles:「可汗」

The context for inventing「皇」was the Qin's wars of unification, where the de-facto rulers of the smaller Warring States all declared themselves to be「王」.「皇」was thus invented to be an overlord title. From Qin unification onwards, any "leader" of "a small state" within China, or in contact with China and admitting themselves into the Sinocentric system, would likely be called a「王」(おう, 왕, vương). In Sinocentric-style vocabulary, they are granted the title of「王」by the Chinese Emperor (封王).


Liu An (劉安),「淮南王」(Prince of Huainan)
Five Kings of Wa,「倭の五王」; see also King of Na gold seal
Sejong the Great (世宗大王),「朝鮮國王」(King of Joseon)

For China,「皇」corresponds to the Chinese Emperor; for Korea, Japan, and Vietnam,「皇」corresponds to their respective「君主」as soon as they are removed from the Sinocentric system.


Naruhito (德仁),「日本天皇」(Emperor of Japan)
Sunjong (純宗　隆熙帝),「大韓帝國皇帝」(Emperor of the Korean Empire)
Lê Lợi (黎利),「大越皇帝」(Emperor of Đại Việt)
To remain within the Sinocentric system, but simultaneously call themselves「皇」, the monarchs would style themselves「王」only when they deal with China in diplomatic missions. Korean Kings were the only ones which styled themselves「王」and dealt with China as「王」; both Japanese and Vietnamese monarchs styled themselves as「皇」in their own regions.