The words marshal, martial sound the same but have different meanings and spellings. Why do marshal, martial sound the same even though they are completely different words?
The answer is simple: marshal, martial are homophones of the English language.
A military officer of the highest rank in some countries.
A field marshal.
A U.S. federal officer of a judicial district who carries out court orders and discharges duties similar to those of a sheriff.
A city law enforcement officer in the United States who carries out court orders.
Of, relating to, or suggestive of war.
Relating to or connected with the armed forces or the profession of arms.
Characteristic of or befitting a warrior.
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").