Xylem and phloem are both plant vascular tissues within vascular plants (seed-bearing plants that include the gymnosperms, angiosperms, ferns, and horsetails). They can be related to the veins and arteries of an animal because they transport materials throughout the plant. Such a system is imperative to have in large plants that must carry materials back and forth between the roots and the leaves of the tree. 

The difference between the two is the direction and substances that each carries. Xylem moves mostly upwards. It carries the nutrients and water absorbed by the roots in the ground to the leaves. Xylem has a secondary function of support. Xylem is what makes up the rings of a tree trunk that can be seen when it is cut.

Phloem, on the other hand, carries the glucose that is made in the leaves during photosynthesis down to the roots.