Short answers, pulled from the story.
A cloak is a loose, almost always sleeveless garment worn over clothing, mostly as outerwear, that serves the same purpose as an overcoat and protects the wearer from the weather. It fastens at the neck or over the shoulder, may have an attached hood, and can form part of a uniform.
The word cloak comes from the Old North French cloque, meaning bell, from Medieval Latin clocca, a travelers' cape literally meaning a bell, named for the garment's bell-like shape. Because of this shared root, cloak is related to the word clock.
Cloaks vary in length from the hip to the ankle, with mid-calf being the normal length. They are almost always sleeveless and generally fasten at the neck or over the shoulder.
Ancient Greeks and Romans wore cloaks such as the himation and the pallium, while Roman magistrates wore the toga as a badge of office. Eminent figures in Kievan Rus' wore the fur-lined Byzantine chlamys, Aztec elites wore the tilmatli, and Aboriginal Australians wore possum-skin and buka cloaks.
The cloak became linked to Dracula because Bela Lugosi wore one in the best-known stage version of the play so his exit through a concealed trapdoor would seem sudden. He kept the cloak for the 1931 Universal Studios film, and the impression was so strong that cloaks came to be equated with Count Dracula in nearly all non-historical depictions.
Cloak and dagger refers to espionage and secretive crimes, suggesting murder from hidden sources, because a cloak keeps a person hidden and conceals a weapon. Cloak and dagger stories are mystery, detective, and crime tales, and the Marvel vigilante duo Cloak and Dagger is named for the phrase.
Opera cloaks are made of quality materials such as wool or cashmere, velvet, and satin, and are worn in full evening dress as a fashion statement or to protect fine fabrics from the elements. Formal cloaks often have expensive colored linings and trimmings such as silk, satin, velvet, and fur.