The words facts, Fax sound the same but have different meanings and spellings. Why do facts, Fax sound the same even though they are completely different words?
The answer is simple: facts, Fax are homophones of the English language.
Plural form of fact.
A fax machine.
A printed page or image transmitted or received by a fax machine.
To transmit (printed matter or an image) by electronic means.
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").