His heir apparent takes his next highest title as a courtesy title. Details are as for duke. He will nearly always be earl of something. His wife is the Countess of Cranthorpe or Lady Cranthorpe, and she will sign herself Desdemona Cranthorpe. As with a duke, the earl's heir will take the next lowest title as a courtesy title, and the heir's son, the next again.
All details are the same.
He will be, for example, Viscount Brummidge, usually known as Lord Brummidge, or just Brummidge. His wife will be known as Lady Brummidge and will sign herself Anne Brummidge. His heir has no special title. All children are known as the honorable. NOTE that the terms baron and baroness are only used in England in the most formal documents.