The words galley, gally sound the same but have different meanings and spellings. Why do galley, gally sound the same even though they are completely different words?
The answer is simple: galley, gally are homophones of the English language.
Nautical A large, usually single-decked medieval ship of shallow draft, propelled by sails and oars and used as a merchant ship or warship in the Mediterranean.
Nautical An ancient Mediterranean seagoing vessel propelled by oars.
Nautical A large rowboat formerly used by British customs officers.
The kitchen of an airliner, ship, or camper.
Alternative spelling of galley.
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").