Gojoseon (pronounced "gaw-jaw-sun," also referred to as Kojoseon, 고조선, 古朝鮮; Proto-Choseon, Ancient Choseon, etc.) was the first Korean kingdom in recorded history. The actual name of the kingdom was simply "Chosun." The reason for calling the ancient Choseon "Gojoseon" is to distinguish this kingdom from other Joseons (or Choseons), all spelled exactly the same in Korean and Chinese. Ilyeon (a Korean Buddhist priest and historian, 1,206 - 1,289 A.D.; 일연 in Korean writing, 一然 in Chinese characters, also transliterated as "Ilyon" in many cases) first used the name "Gojoseon" in his Korean history book Samgukyusa (or "Samguk Yusa," 삼국유사; 三國遺事, originally written using Chinese characters) to refer to the ancient Joseon at its founding and at its zenith in contrast with a much smaller later remnant of Gojoseon, which he called "Wimanjoseon" (i.e., the Joseon of the Wiman family). Generally, Choseon (Joseon, Chosun, 조선; 朝鮮) without a prefix means later Choseon (1388 - 1897; succeeded by the Korean Empire (대한제국; 1897 - 1910)) unless the antiquity of the time is clearly implied in the context. To facilitate scholastic discussions, there is a practical need to differentiate the Chosun of antiquity from the later Choseon (Joseon, Chosun, 조선; 朝鮮). The convention devised by Ilyeon has been almost universally accepted now. Thus, the prefix "Go (고, 古)," meaning "antiquity," is attached to the word "Joseon (조선, 朝鮮)" to form the word "Gojoseon" and identity Joseon of antiquity. Obviously, Gojoseon was not an "ancient" country but a currently-existing country to the people at that time its existence. To the people of Gojoseon, "Joseon" was the name for their own country.
I think Korean history is fascinating