When I think of the friendships that come to mind in the book, particularly those between Wilbur and Charlotte herself, a variety of ideas about friendship come up.

The first is the idea that friendship needn't be limited according to what group one is in, in this case even by what species one belongs to.  A pig can be friends with a spider and that friendship can mean a great deal and both can learn from each other.

Another is the way that friends are willing to sacrifice for each other, and the way that a deep and personal friendship is something that can last even after one of the friends dies or moves on.