The words bad, bade sound the same but have different meanings and spellings. Why do bad, bade sound the same even though they are completely different words?
The answer is simple: bad, bade are homophones of the English language.
Not achieving an adequate standard; poor: a bad concert.
Evil; sinful.
Vulgar or obscene: bad language.
Informal Disobedient or naughty: bad children.
A past tense of bid.
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").