What is the difference between amber and tree sap?

Amber is fossilized tree resin, which in and of itself is entirely different from tree sap. While sap is actually the sugar stored by the tree in its xylem and phloem, resin is the liquid stored on the outer cells of trees. Similar to human blood in function, when an appendage of a tree, such as a branch, is cut, resin rushes to the wound and clogs it. This is much like how blood clots to seal wounds.