The words courier, currier sound the same but have different meanings and spellings. Why do courier, currier sound the same even though they are completely different words?
The answer is simple: courier, currier are homophones of the English language.
A messenger, especially one on official diplomatic business.
A spy carrying secret information.
A personal attendant hired to make arrangements for a journey.
An employee of a travel agency serving as a guide for tourists.
One that prepares tanned hides for use.
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").