The words gorilla, guerilla sound the same but have different meanings and spellings. Why do gorilla, guerilla sound the same even though they are completely different words?
The answer is simple: gorilla, guerilla are homophones of the English language.
The largest of the anthropoid apes (Gorilla gorilla) native to the forests of equatorial Africa, having a stocky body and coarse, dark brown or black hair.
Slang A brutish man.
Slang A thug.
Variant of guerrilla.
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").