The forebrain is composed of the diencephalon and the telencephalon.

You learned that the forebrain is made from the cells that develop from that most rostral of the three original brain vesicles. Then you saw that this forebrain vesicle splits into two vesicles, the diencephalon and the telencephalon for the 5 vesicle stage embryo. The telencephalic vesicle grows like mad, so much so that it overtakes the rest of the brain in size; the diencephalic vesicle grows at a more normal pace, much like the hindbrain vesicles do.

Here you will learn more about the diencephalon and telencephalon. You will get less information about the diencephalon than the telencephalon, since it is smaller and subserves fewer functions.