The words cruel, crewel sound the same but have different meanings and spellings. Why do cruel, crewel sound the same even though they are completely different words?
The answer is simple: cruel, crewel are homophones of the English language.
Loosely twisted worsted yarn used for fancywork and embroidery.
Disposed to inflict pain or suffering.
Causing suffering; painful.
Definitions 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").