* Brown is the broad category: Brown is the general color descriptor, encompassing a wide range of shades.
* Brunette is a specific term for hair: Brunette specifically refers to a person with brown hair.
* Subtleties within Brown: Brown hair can range from very light brown (often called "light brown" or even "dishwater blonde") to very dark brown (almost black). Brunette covers a range of brown hair colors, usually excluding the very lightest shades and typically not including hair that is so dark it appears black.
In short: All brunettes have brown hair, but not all brown hair would necessarily be described as "brunette".
Think of it this way:
* "Dog" is a general term.
* "Poodle" is a specific type of dog.
Similarly:
* "Brown" is a general hair color.
* "Brunette" is a specific term for brown hair.