The words flew, flu, flue sound the same but have different meanings and spellings. Why do flew, flu, flue sound the same even though they are completely different words?
The answer is simple: flew, flu, flue are homophones of the English language.
Past tense of fly1.
Informal Influenza.
A pipe, tube, or channel for conveying hot air, gas, steam, or smoke, as from a furnace or fireplace to a chimney.
Music An organ pipe sounded by means of a current of air striking a lip in the side of the pipe and causing the air within to vibrate. Also called labial.
Music The lipped opening in such a pipe.
A fishing net.
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").