The words tacks, tax sound the same but have different meanings and spellings. Why do tacks, tax sound the same even though they are completely different words?
The answer is simple: tacks, tax are homophones of the English language.
Plural form of tack.
Third-person singular simple present indicative form of tack.
A contribution for the support of a government required of persons, groups, or businesses within the domain of that government.
A fee or dues levied on the members of an organization to meet its expenses.
A burdensome or excessive demand; a strain.
To place a tax on (income, property, or goods).
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.
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").