The words timber, timbre sound the same but have different meanings and spellings. Why do timber, timbre sound the same even though they are completely different words?
The answer is simple: timber, timbre are homophones of the English language.
Trees or wooded land considered as a source of wood.
Wood used as a building material; lumber.
A dressed piece of wood, especially a beam in a structure.
Nautical A rib in a ship's frame.
The combination of qualities of a sound that distinguishes it from other sounds of the same pitch and volume.
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").