Xylem and phloem are the tubes in the stems of plants that transport water, nutrients, and other stuff. Xylem make up the middle of the stem and transport water and nutrients; xylem are made up of dead cells .
Phloem, on the other hand, are nearer the surface of the stem; they make up the inner bark of trees. Phloem are made of living cells and they generally transport the plant's sap. This isn't a hard-and-fast rule, though. Maple syrup, for example, is made from sap extracted from the xylem of a maple tree.