The words warn, worn sound the same but have different meanings and spellings. Why do warn, worn sound the same even though they are completely different words?
The answer is simple: warn, worn are homophones of the English language.
To notify or make aware in advance of something, especially of possible danger or misfortune.
To make known (a warning).
To advise or caution (someone) about a course of action.
To notify (a person) to go or stay away.
Affected or damaged by wear or use.
Showing the wearing effects of overwork, worry, or suffering.
Impaired or otherwise affected by wear or use.
Definitions from The American Heritage® Dictionary of the English Language, 5th Edition., from The Century Dictionary. 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").