The words disc, disk sound the same but have different meanings and spellings. Why do disc, disk sound the same even though they are completely different words?
The answer is simple: disc, disk are homophones of the English language.
Variant of disk.
A thin, flat, circular object or plate.
Something resembling such an object: The moon's disk was reflected in the pond.
The disk used in a disc brake.
A disk used on a disk harrow.
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").