The words cirrous, cirrus, serous sound the same but have different meanings and spellings. Why do cirrous, cirrus, serous sound the same even though they are completely different words?
The answer is simple: cirrous, cirrus, serous are homophones of the English language.
Pertaining to cirri; having filaments, fibrous.
Pertaining to cirrus clouds.
A high-altitude cloud composed of narrow bands or patches of thin, generally white, fleecy parts.
Biology A tendril or similar part.
Biology A slender flexible appendage, such as the fused cilia of certain protozoans.
Containing, secreting, or resembling serum.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, from The American Heritage® Dictionary of the English Language, 4th Edition 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").