The words roc, rock sound the same but have different meanings and spellings. Why do roc, rock sound the same even though they are completely different words?
The answer is simple: roc, rock are homophones of the English language.
A mythical bird of prey having enormous size and strength.
Relatively hard, naturally formed mineral or petrified matter; stone.
A relatively small piece or fragment of such material.
A relatively large body of such material, as a cliff or peak.
A naturally formed aggregate of mineral matter constituting a significant part of the earth's crust.
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").