The words dear, deer sound the same but have different meanings and spellings. Why do dear, deer sound the same even though they are completely different words?
The answer is simple: dear, deer are homophones of the English language.
Loved and cherished: my dearest friend.
Greatly valued; precious: lost everything dear to them.
Highly esteemed or regarded. Used in direct address, especially in salutations: Dear Lee Dawson.
Any of various hoofed ruminant mammals of the family Cervidae, characteristically having deciduous antlers borne chiefly by the males. The deer family also includes the elk, moose, caribou, and reindeer.
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").