The words retard, retarred sound the same but have different meanings and spellings. Why do retard, retarred sound the same even though they are completely different words?
The answer is simple: retard, retarred are homophones of the English language.
To cause to move or proceed slowly; delay or impede.
To be delayed.
A slowing down or hindering of progress; a delay.
Music A slackening of tempo.
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").