The words cheap, cheep sound the same but have different meanings and spellings. Why do cheap, cheep sound the same even though they are completely different words?
The answer is simple: cheap, cheep are homophones of the English language.
Relatively low in cost; inexpensive or comparatively inexpensive.
Charging low prices: a cheap restaurant.
Obtainable at a low rate of interest. Used especially of money.
Devalued, as in buying power: cheap dollars.
A faint, shrill sound like that of a young bird; a chirp.
To make a faint, shrill sound or sounds; chirp.
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").