The words cannon, canon sound the same but have different meanings and spellings. Why do cannon, canon sound the same even though they are completely different words?
The answer is simple: cannon, canon are homophones of the English language.
A large mounted weapon that fires heavy projectiles. Cannon include guns, howitzers, and mortars.
The loop at the top of a bell by which it is hung.
A round bit for a horse.
Zoology The section of the lower leg in some hoofed mammals between the hock or knee and the fetlock, containing the cannon bone.
An ecclesiastical law or code of laws established by a church council.
A secular law, rule, or code of law.
An established principle: the canons of polite society.
A basis for judgment; a standard or criterion.
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").