The words guest, guessed sound the same but have different meanings and spellings. Why do guest, guessed sound the same even though they are completely different words?
The answer is simple: guest, guessed are homophones of the English language.
Simple past tense and past participle of guess.
Provided for guests: guest rooms.
One who is a recipient of hospitality at the home or table of another.
One to whom entertainment or hospitality has been extended by another in the role of host or hostess, as at a party.
One who pays for meals or accommodations at a restaurant, hotel, or other establishment; a patron.
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").