The big blackish lines as well as the smaller silvery lines are all veins in the leaf. The largest black line is the leaf's single "midvein," running from the petiole to the leaf tip, the smaller blackish lines branching off the midvein are "secondary veins," and all the slender, silvery lines are smaller veins. All the veins, from midvein to the smallest silvery one, serve the same functions.
You can see that the network is very extensive. If the veins in a single elm leaf were placed end to end they would be more than 700 feet long (210 m)!
The word "vein" is a general one. If we were in a botany lab studying leaf anatomy, we might refer to leaf veins as vascular bundles. The "bundle" part of that term arises from the fact that each vein is actually a bundle of these two different kinds of conducting tissue:
We also meet xylem and phloem on our twig page.