The words cel, cell, sell sound the same but have different meanings and spellings. Why do cel, cell, sell sound the same even though they are completely different words?
The answer is simple: cel, cell, sell are homophones of the English language.
A piece of celluloid on which has been drawn a frame of an animated film.
A narrow confining room, as in a prison or convent.
A small enclosed cavity or space, such as a compartment in a honeycomb or within a plant ovary or an area bordered by veins in an insect's wing.
Biology The smallest structural unit of an organism that is capable of independent functioning, consisting of one or more nuclei, cytoplasm, and various organelles, all surrounded by a semipermeable cell membrane.
Architecture See web.
To exchange or deliver for money or its equivalent.
To offer for sale, as for one's business or livelihood: The partners sell textiles.
To give up or surrender in exchange for a price or reward: sell one's soul to the devil.
To be purchased in (a certain quantity); achieve sales of: a book that sold a million copies.
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.