The words we'd, weed sound the same but have different meanings and spellings. Why do we'd, weed sound the same even though they are completely different words?
The answer is simple: we'd, weed are homophones of the English language.
Contraction of we had.
Contraction of we should.
Contraction of we would.
A plant considered undesirable, unattractive, or troublesome, especially one growing where it is not wanted, as in a garden.
Rank growth of such plants.
A water plant, especially seaweed.
The leaves or stems of a plant as distinguished from the seeds: dill weed.
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").