Someone who's aloof isn't warm and friendly, instead being distant and reserved. That emotionally cold and detached fellow who keeps to himself, drinking espresso and reading French philosophy, would best be described as aloof.
In Middle English, aloof was originally a nautical term; the loof (now spelled luff) is the windward side of a ship. Smart sailors wanting to avoid a hazard on the leeward side would give the order, "A loof!" From this command we get the idea of steering clear of something (or someone). In modern usage the word has taken on a negative connotation: an aloof person is often considered cold or snobby.