The words billed, build sound the same but have different meanings and spellings. Why do billed, build sound the same even though they are completely different words?
The answer is simple: billed, build are homophones of the English language.
Simple past tense and past participle of bill.
Having a specified kind of bill (beak or beak-like projection).
To form by combining materials or parts; construct.
To order, finance, or supervise the construction of: The administration built several new housing projects.
To develop or give form to according to a plan or process; create: build a nation; built a successful business out of their corner grocery store.
To increase or strengthen by adding gradually to: money building interest in a savings account; build support for a political candidate.
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").