The words marc, mark, marque sound the same but have different meanings and spellings. Why do marc, mark, marque sound the same even though they are completely different words?
The answer is simple: marc, mark, marque are homophones of the English language.
The pulpy residue left after the juice has been pressed from grapes, apples, or other fruits.
Brandy distilled from grape or apple residue.
A visible trace or impression, such as a line or spot.
A sign, such as a cross, made in lieu of a signature.
A written or printed symbol used for punctuation; a punctuation mark.
A number, letter, or symbol used to indicate various grades of academic achievement: got a mark of 95 instead of 100.
A model or brand of a manufactured product, especially an automobile.
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").