This coin features Admiral Yi Sun-sin on the obverse. This man, who lived from 1545 to 1598, was a commander of the naval fleet under the the rulers of Joseon Dynasty.
Despite the fact that he had no prior naval training, Admiral Yi was never defeated at sea nor lost a single ship under his command to enemy action, and military historians have placed him on par with Admiral Horatio Nelson as one of the greatest naval commanders in history. [source]
Even in death, he was a staunch strategist. His purported last words after being mortally wounded in battle were: "The battle is at its height. Beat my war drums. Do not announce my death." 
To the left and right of the portrait is written the denomination of the coin "100 WON" in the Hangul lettering of the Korean language. Across the bottom of the coin is "BANK OF KOREA." Contrary to convention in Western culture, there is no privy mark to honor the engraver, Kang Bak.
The reverse is especially simple and beautiful: the denomination "100" appears above the year-of-minting, surrounded by a lovely geometric design which repeats, encircling the coin. I really like the artistry of the reverse of this coin, and both sides are uncluttered and elegant in appearance.
The face-value of this copper-nickel coin is only about USD $0.08 today, and the collector value is USD $1.00, roughly. This coin is still in circulation, so is valid legal tender in Korea.