Muggles' Guide to Harry Potter/Magic/Rictusempra

Overview
Rictusempra is a jinx which attempts to disable a witch or wizard by tickling him or her.

Extended Description
Rictusempra, the Perpetual Tickling charm, which apparently can be stopped by the Finite Incantatem charm, was used by Harry against Malfoy in the Duelling Club. The intent was, of course, to tickle Malfoy to the point that he would be unable to jinx Harry in return. It was ineffectual, however, as despite being tickled, Malfoy was able to cast Tarantallegra at Harry afterwards.

It should be noted that in the films and in the video games, Rictusempra is used as a much more powerful jinx, possibly along the lines of Stupefy. This use is not in line with the spell description in the books.