The words bird, burd, burred sound the same but have different meanings and spellings. Why do bird, burd, burred sound the same even though they are completely different words?
The answer is simple: bird, burd, burred are homophones of the English language.
Chiefly British Slang A young woman.
To observe and identify birds in their natural surroundings.
Any of various warm-blooded, egg-laying, feathered vertebrates of the class Aves, having forelimbs modified to form wings.
Such an animal hunted as game.
poetic Maiden, young woman
Simple past tense and past participle of burr.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License 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").