Italian Renaissance began in Italy during the 14th century and lasted until the 16th century; Northern Renaissance occurred in countries North of Italy. Italian Renascentists' ideas began spreading throughout Europe only during the late 15th century.
To approach the topic, let's look at two paintings: Michelangelo's 'The Creation of Adam' (Italian Renaissance) and Albrecht Durer's 'Adam and Eve' (Northern Renaissance)
There are some similarities: both have a Biblical theme and both of them depict and explore the shape of the human body.
A defining feature of Renaissance thinking is the belief in human beings as not only rational, sentient and able of making moral judgements, but also as the centre of God's creation. This explains why Renascentists painting both depicts Biblical themes and the perfect human body. (As an exercise, can you think of other paintings in which you see the same elements: Biblical theme and emphasis on the human body?)
In order to understand how Italian and Northern Renaissance were different, we need to turn to ideas as well. Italy was mostly Catholic, while the German lands and the Netherlands began to embrace Protestantism; even before Martin Luther, there was some degree of questionning the rituals of the Catholic Church (made possible by being less under Papal authority than the Italians).
Now, if you look at the two paintings again, in the Michelangelo one, the emphasis falls on Adam rather than on God; the painting seems to be about 'Adam receiving the spark of life from God' rather than 'God giving the spark of life to Adam': the human being, as rational and sentient, is bearer of a spark of divinity. (You might even notice that the shapes behind God form the anatomically correct picture of a brain: what is Michelangelo trying to say with this? That the human mind is God-like? That God is merely a creation of the human mind?) The Divine figure is very obviously present, but the emphasis falls on the human.
On the other hand, if you look at Durer's Adam and Eve, you don't see God himself depicted. Adam and Eve's bodies are, in terms of proportion and body type, quite similar to Michelangelo's Adam; they are ideal forms by the same beauty standards. But the colors are slightly more muted, their nudity is slightly concealed with leaves; they are depicted holding not God's spark of divinity, but the apple that brought about their downfall. They are not as glorious as Michelangelo's Adam.
You could associate this with different kinds of theology and religious worship: the belief in the inherent goodness of human beings, or their inherent sinfulness; the belief in the worship of icons as an important part of Christianity or as idolatry; and then map those differences along the lines of the cultural/theological line of thinking that caused the separation of Protestantism from Catholicism.
Another aspect you could look at: 'The Creation of Adam' is a fresco on the Sistine Chapel ceiling. Durer's 'Adam and Eve' consists of a pair of oil-on-panel paintings; he had completed a copper engraving on the same theme two years before. Unlike Italian Renaissance, Northern Renaissance shows a higher diversity of visual art forms, such as engravings and woodcuts. As Protestantism gained ground, paintings were much less likely to be created specifically for places of worship, as unlike Catholics some Protestants believed that worshipping icons/religious imagery is idolatry.