The words hoo, who sound the same but have different meanings and spellings. Why do hoo, who sound the same even though they are completely different words?
The answer is simple: hoo, who are homophones of the English language.
hurrah; an exclamation of triumphant joy
Used to grab the attention of others.
she
how
What or which person or persons: Who left?
Used as a relative pronoun to introduce a clause when the antecedent is a person or persons or one to whom personality is attributed: the visitor who came yesterday; our child, who is gifted; informed sources who denied the story.
The person or persons that; whoever: Who believes that will believe anything.
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").