The primary difference between the two is that while the works of both contained religious themes, the Italian Renaissance was more devoted to classical Greco-Roman overtones. The Northern Renaissance had a much more pronounced religous element. Among the famous art works of the Northern Renaissance were Albrecht Durer's Praying Hands, Jan Van Eck's Virgin Mary and Child, and Jerome Bosch's Death and the Miser. All are distinctly religious, but none have the Greco-Roman themes of works of the Italian masters, such as Michaelangelo (Statue of David and the ceiling of the Sistine Chapel) or Donatello (Statue of David.) The difference is understandable since Italy was the home of classical Roman culture.
The Northern Renaissance is the term typically given to the art in the countries north of Italy, while the Southern Renaissance is the term that really could be called the Italian Renaissance. Two main differences are details and composition. Northern Renaissance paintings are full of things to look at besides the person(s) who is the subject of the work while Italian/Southern Renaissance paintings tended to be portraits done with not much else, if anything, depicted besides the subject. The Italians, particularly DaVinci, were interested in compositions in mathematical proportions and heavily influenced by the sculptures of ancient Greece and Rome, while Northern Renaissance painters seemed to give equal importance to the various and assorted items in their backgrounds, often landscapes.