The words ferrule, ferule sound the same but have different meanings and spellings. Why do ferrule, ferule sound the same even though they are completely different words?
The answer is simple: ferrule, ferule are homophones of the English language.
A metal ring or cap placed around a pole or shaft for reinforcement or to prevent splitting.
A bushing used to secure a pipe joint.
An instrument, such as a cane, stick, or flat piece of wood, used in punishing children.
To punish with a ferule.
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").