The words earn, erne, urn sound the same but have different meanings and spellings. Why do earn, erne, urn sound the same even though they are completely different words?
The answer is simple: earn, erne, urn are homophones of the English language.
To gain especially for the performance of service, labor, or work: earned money by mowing lawns.
To acquire or deserve as a result of effort or action: She earned a reputation as a hard worker.
To yield as return or profit: a savings account that earns interest on deposited funds.
spurs To gain a position through hard work and the accumulation of experience, often in the face of difficulties.
Any of several sea eagles, especially Haliaeetus albicilla, of Europe.
A vase of varying size and shape, usually having a footed base or pedestal.
A closed metal vessel having a spigot and used for warming or serving tea or coffee.
Botany The spore-bearing part of a moss capsule.
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").