The words tahr, tar sound the same but have different meanings and spellings. Why do tahr, tar sound the same even though they are completely different words?
The answer is simple: tahr, tar are homophones of the English language.
Any of several goatlike mammals of the genus Hemitragus of mountainous regions of Asia, having curved horns and a shaggy coat.
A dark, oily, viscous material, consisting mainly of hydrocarbons, produced by the destructive distillation of organic substances such as wood, coal, or peat.
A solid residue of tobacco smoke containing byproducts of combustion.
To coat with or as if with tar.
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.