The title of viscount originated in the Holy Roman Empire as the deputy to a count, thus vice-count. In Norman England a viscount was a judicial honorific that was used to refer to the sheriff of a county. It was only turned into a hereditary noble title in 1440 by Henry VI, making it a relatively late addition to the peerage.
The Viscount Hereford is the premier viscount of England; the title was created in 1550 for Walter Devereux.
A viscount’s wife is a viscountess; she is address as “lady”; a viscount is addressed as “lord.”
A British viscount wears a coronet (a gold-plated metal circlet with faux jewels pressed from the metal) that features 16 silver balls (known as “pearls”) touching each other. See above for an image of a viscount’s coronet. The coronet is worn only at coronations.
Image of a viscount’s coronet by Sodacan / Wikipedia. Image licensed under the Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic, 2.0 Generic, and 1.0 Generic licenses.