The words missal, missile sound the same but have different meanings and spellings. Why do missal, missile sound the same even though they are completely different words?
The answer is simple: missal, missile are homophones of the English language.
Roman Catholic Church A book containing all the prayers and responses necessary for celebrating the Mass throughout the year.
A prayer book.
An object or weapon that is fired, thrown, dropped, or otherwise projected at a target; a projectile.
A guided missile.
A ballistic missile.
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").