The words cedar, ceder, seeder sound the same but have different meanings and spellings. Why do cedar, ceder, seeder sound the same even though they are completely different words?
The answer is simple: cedar, ceder, seeder are homophones of the English language.
Any of several Old World evergreen coniferous trees of the genus Cedrus, having stiff needles on short shoots and large erect seed cones with broad deciduous scales.
Any of several other evergreen coniferous trees or shrubs, such as the Alaska cedar, incense cedar, or red cedar.
The durable aromatic wood of any of these plants, especially that of the red cedar, often used to make chests.
A machine or an implement used for planting seeds.
A machine or implement used to remove the seeds from fruit.
One that seeds clouds.
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").