The words cede, seed sound the same but have different meanings and spellings. Why do cede, seed sound the same even though they are completely different words?
The answer is simple: cede, seed are homophones of the English language.
To surrender possession of, especially by treaty. See Synonyms at relinquish.
To yield; grant: The debater refused to cede the point to her opponent.
Sperm; semen.
A seed oyster or oysters; spat.
A ripened plant ovule containing an embryo.
A propagative part of a plant, as a tuber or spore.
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.
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").