The words you, ewe, yew sound the same but have different meanings and spellings. Why do you, ewe, yew sound the same even though they are completely different words?
The answer is simple: you, ewe, yew are homophones of the English language.
A female sheep, especially when full grown.
Any of several poisonous evergreen trees or shrubs of the genus Taxus, having scarlet cup-shaped arils and flat needles that are dark green above and yellowish below.
The wood of any of these trees, especially the durable, fine-grained wood of the Old World species Taxus baccata, used in cabinetmaking and for archery bows.
Used to refer to the one or ones being addressed: I'll lend you the book. You shouldn't work so hard. See Regional Notes at you-all, you-uns.
Used to refer to an indefinitely specified person; one: You can't win them all.
Nonstandard Used reflexively as the indirect object of a verb: You might want to get you another pair of shoes. See Note at me.
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.