The words calk, caulk sound the same but have different meanings and spellings. Why do calk, caulk sound the same even though they are completely different words?
The answer is simple: calk, caulk are homophones of the English language.
A pointed extension on the toe or heels of a horseshoe, designed to prevent slipping.
A spiked plate fixed on the bottom of a shoe to prevent slipping and preserve the sole.
Variant of caulk.
To make watertight or airtight by filling or sealing: caulk a pipe joint; caulked the cracks between the boards with mud.
Nautical To make (a boat) watertight by packing seams with a waterproof material, such as oakum or pitch.
To apply caulking: caulked all around the window frame.
Caulking.
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").