"The Canterville Ghost" is described as a comico-horror story because it has elements which both amuse and scare the reader.
Beginning with comedy, there are a number of humorous elements in the story. Washington Otis's cleaning of the blood stain with Pinkerton's Stain Removal in Chapter One, for example, and Mrs Otis's offer of a tincture in Chapter Three, are both designed to make the reader laugh. The tricks played on the ghost by the twins are also amusing. They attack him with pea-shooters, for instance, and leave nut-shells on the floor for him to stand on.
On the other hand, there are plenty of scary references in "The Canterville Ghost," too. When the Otis's buy Canterville Chase, for example, Lord Canterville asserts that the house is haunted and relates a frightening tale which explains his reason for selling. His great-aunt, the Dowager Duchess, was frightened "into a fit" after seeing skeleton hands on her shoulders while she was dressing for dinner. Similarly, Lady Canterville found herself unable to sleep at night because of the strange noises emanating from the corridor and the library.
The house's murderous history is another scary element in the story. In 1575, Sir Simon murdered his wife, Eleanore, and this act is immortalized by the blood stain in the library. This example is especially important because it demonstrates how Wilde combines comedy and horror to great effect. While the murder is itself a frightening act, Sir Simon's motivations are so trivial that they become funny to the reader:
My wife was very plain, never had my ruffs properly starched, and knew nothing about cookery.
In sum, then, Wilde is able to make the reader laugh and feel fear and, in some cases, feel both emotions at the same time.