The words coulee, coolie, coolly sound the same but have different meanings and spellings. Why do coulee, coolie, coolly sound the same even though they are completely different words?
The answer is simple: coulee, coolie, coolly are homophones of the English language.
Offensive An unskilled Asian laborer.
In a cool, unpanicked or collected manner.
Western U.S. A deep gulch or ravine with sloping sides, often dry in summer.
Louisiana & Southern Mississippi A streambed, often dry according to the season.
Louisiana & Southern Mississippi A small stream, bayou, or canal.
Upper Midwest A valley with hills on either side.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").