"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.