The Four Species - Arba'at Haminim

"And you shall take for yourselves on the first day [of Sukkot], the fruit of the beautiful [citron] tree, tightly bound branches of date palms, the branch of the braided [myrtle] tree, and willows of the brook, and you shall rejoice before the Lord your God seven days." (Leviticus 23:40)

The Four Species are the Lulav, Hadass, Arava and Etrog of which the first three are bound together. The waving of the Four Species is a biblical commandment and many traditions and explanations for why these particular species were chosen are described in Rabbinic literature.
Beautifying the commandment is considered of utmost importance and people spend large amounts of money to acquire the most perfect etrog, the straightest lulav, and the freshest hadass and aravah.
Together, they signify unity, the four different types of Jew. Each also alludes to different parts of the human body.
Etrog Etrog
The etrog is the fruit of a citron tree, Hadar in Hebrew, and has both a good taste and a good smell, symbolizing those who study Torah and also do good deeds. It is also described as alluding to the heart. The root of the Hebrew name, hadar, means beauty.
Lulav Lulav
The lulav is a green, closed frond from a date palm tree. The dates have a sweet taste but no smell, symbolizing those who study Torah but do not do good deeds. The date palm is considered the king of trees and has only one heart whereas other trees have many hearts (from which branches grow). It is also described as alluding to the spine.
Hadass Hadass
The hadassim (three are used) are leaf bearing boughs, from the myrtle tree. The hadass has a good smell but no taste, symbolizing those who do good deeds but do not study the Torah. It is considered a sign of success in life and alludes to the eyes, which the myrtle leaf is said to resemble.
Arava Arava
Aravot (two branches are used) are branches with leaves from the willow tree. The arava has neither taste nor smell, symbolizing those who do not study Torah or do good deeds. It is also described as alluding to the mouth, which the willow leaf resembles.