The words cask, casque sound the same but have different meanings and spellings. Why do cask, casque sound the same even though they are completely different words?
The answer is simple: cask, casque are homophones of the English language.
A sturdy cylindrical container for storing liquids; a barrel.
The quantity that such a container can hold.
A helmet, especially an ornate visorless headpiece of the 16th century.
Zoology A helmetlike structure or protuberance.
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").