What is a Lulav and Etrog?

One of the special Sukkot mitzvot is the mitzvah of the "Four Kinds" - the lulav, etrog, myrtle and willow. This is a mitzvah like any other mitzvah of the Torah, but it is a very significant one and symbolic of unity and harmony. When the blessings are recited over them, it is customary to wave them to all the four winds and also upward and downward, signifying that G‑d is everywhere.