The words sewer, sower sound the same but have different meanings and spellings. Why do sewer, sower sound the same even though they are completely different words?
The answer is simple: sewer, sower are homophones of the English language.
An artificial, usually underground conduit for carrying off sewage or rainwater.
A medieval servant who supervised the serving of meals.
One that sews: a sewer of fine clothing.
One who sows.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License 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").