Handedness is a term that refers to having a preference for using, and greater fine motor skill in, one hand over the other. In Muggles, handedness is usually defined by which hand is used to write, while handedness in wizards and witches is more commonly defined by which hand is used to hold the wand. The majority of humans are right-handed.
Harry Potter is shown on the books cover to be left handed with is wand but is mentioned in the chamber of secrets that he has problems eating with his left hands. Which suggests he's Mixed-Handed.