The words sailer, sailor sound the same but have different meanings and spellings. Why do sailer, sailor sound the same even though they are completely different words?
The answer is simple: sailer, sailor are homophones of the English language.
That which sails; a boat.
One who serves in a navy or works on a ship.
One who travels by water.
A low-crowned straw hat with a flat top and flat brim.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, 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").