The words resister, resistor sound the same but have different meanings and spellings. Why do resister, resistor sound the same even though they are completely different words?
The answer is simple: resister, resistor are homophones of the English language.
A person who resists going along with others in a common course of action.
A person who fights against a government.
A device that bars the passage of an electric charge between two points.
A device used to control current in an electric circuit by providing resistance.
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").