The words ferri-, ferry sound the same but have different meanings and spellings. Why do ferri-, ferry sound the same even though they are completely different words?
The answer is simple: ferri-, ferry are homophones of the English language.
Iron, especially ferric iron: ferricyanide.
Nautical To transport (people, vehicles, or goods) by boat across a body of water.
Nautical To cross (a body of water) by a ferry.
To deliver (a vehicle, especially an aircraft) under its own power to its eventual user.
To transport (people or goods) by vehicle, especially by aircraft.
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").