The words it will [ 'twill ], twill sound the same but have different meanings and spellings. Why do it will [ 'twill ], twill sound the same even though they are completely different words?
The answer is simple: it will [ 'twill ], twill are homophones of the English language.
A fabric with diagonal parallel ribs.
The weave used to produce such a fabric.
To weave (cloth) so as to produce a pattern of diagonal parallel ribs.
A fabric with diagonal parallel ribs.
The weave used to produce such a fabric.
To weave (cloth) so as to produce a pattern of diagonal parallel ribs.
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").