When we talk about the Northern Renaissance, what we actually mean is "Renaissance happenings that occurred within Europe, but outside of Italy." Because the most innovative art was created in France, the Netherlands and Germany during this time, and because all of these places are north of Italy, the "Northern" tag has stuck.
What were the differences between the Northern and Italian Renaissances?
Geography aside, there were some significant differences between the Italian Renaissance and the Northern Renaissance. For one thing, the north held on to Gothic (or "Middle Ages") art and architecture with a tighter, longer grip than did Italy. (Architecture, in particular, remained Gothic until well into the 16th century.) This isn't to say that art wasn't changing in the north - in many instances it kept apace with Italian doings. The Northern Renaissance artists, however, were scattered about and few in number initially (very unlike their Italian counterparts).
The north had fewer centers of free commerce than did Italy. Italy, as we saw, had numerous Duchies and Republics which gave rise to a wealthy merchant class that often spent considerable funds on art. This wasn't the case in the north. In fact, the only notable similarity between northern Europe and, say, a place like Florence, lay in the Duchy of Burgundy.
Burgundy, until 1477, encompassed a territory from present-day middle France northward (in an arc) to the sea, and included Flanders (in modern Belgium) and parts of the current Netherlands. It was the only individual entity standing between France and the enormous Holy Roman Empire. Its Dukes, during the last 100 years it existed, were given monikers of "the Good," "the Fearless" and "the Bold" (although apparently the last "Bold" Duke wasn't quite bold enough, as Burgundy was absorbed by both France and the Holy Roman Empire at the end of his reign...but, I digress...)
The Burgundian Dukes were excellent patrons of the arts, but the art they sponsored was different from that of their Italian counterparts. Their interests were along the lines of illuminated manuscripts, tapestries and furnishings (they owned quite a few castles, these Dukes). Things were different in Italy, where patrons were more keen on paintings, sculpture and architecture.
In the broader scheme of things, the social changes in Italy were inspired, as we've seen, by Humanism. Italian artists, writers and philosophers were driven to study Classical antiquity and explore man's supposed capacity for rational choice. They believed that Humanism led to more dignified and worthy humans. In the north (possibly in part because the north did not have works of antiquity from which to learn), change was brought about by a different rationale. Thinking minds in the north were more concerned with religious reform, feeling that Rome (from whom they were physically distanced) had strayed too far from Christian values. In fact, as northern Europe became more openly rebellious over the authority of the Church, art took a decidedly secular turn.
Additionally, Renaissance artists in the north took a different approach to composition than did Italian artists. Where an Italian artist was apt to consider scientific principles behind composition (i.e., proportion, anatomy, perspective) during the Renaissance, northern artists were more concerned with what their art looked like. Color was of key importance, above and beyond form. And the more detail a northern artist could cram into a piece, the happier he was. Close inspection of Northern Renaissance paintings will show the viewer numerous instances where individual hairs have been carefully rendered, along with every single object in the room including the artist himself, distantly inverted in a background mirror.
Finally, it's important to note that northern Europe enjoys different geophysical conditions than does (most of) Italy. For example, there are lots of stained glass windows in northern Europe partly for the practical reason that people living there have more need of barriers against the elements.
Italy, during the Renaissance (and, of course, beyond) produced some fabulous egg tempera paintings and frescoes, along with glorious marble statuary. There's an excellent reason the north isn't known for its frescoes: The climate isn't conducive to curing them. (Ditto for egg tempera painted on wood.) The north developed the chemistry of oil paints for this very reason. Artists needed a medium that would dry (however slowly) and last. Italy produced marble sculptures because, by gum, it has marble quarries. You'll note that Northern Renaissance sculpture is, by and large, worked in wood. (Care to hazard a guess on which area had a lot of trees and hardly any marble quarries?)