The words caste, cast sound the same but have different meanings and spellings. Why do caste, cast sound the same even though they are completely different words?
The answer is simple: caste, cast are homophones of the English language.
The number thrown.
To draw (lots).
To throw (something, especially something light): The angler cast the line.
To throw with force; hurl: waves that cast driftwood far up on the shore. See Synonyms at throw.
Any of the hereditary, endogamous social classes or subclasses of traditional Hindu society, stratified according to Hindu ritual purity, especially the Brahman, Kshatriya, Vaisya, and Sudra castes.
A social class separated from others by distinctions of hereditary rank, profession, or wealth.
A social system or the principle of grading society based on castes.
The social position or status conferred by a system based on castes: lose caste by doing work beneath one's station.
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").