"And, you shall take, on the first day (of Sukkot) a fruit of a citric tree (etrog), branches of palm trees (lulab), branch of myrtle (hadas) and willows of the brook (araba), and you shall rejoice before HaShem, your G-d, for seven days". (vaikra 23).
We are commanded to take four species during the seven days of Sukkot. Two blessings are said the first day, 'al netilat lulab and she'hecheeyanu. One berakha, 'al netilat lulab, is said during the other six days.
One lulab, one etrog, two arabot, and three hadasim are taken together each time.
The etrog with the left hand and the other three with the right hand.
The four species are not taken on Shabbat, even when the first day falls on Shabbat. This Mitzva is perfomrmed during the day but not at night.
On the first day of Sukkot, a person must be careful to take and recite the blessing over his own lulab and etrog. During the other six days, the species can be borrowed from somebody else.
The etrog or Citron, resembles in its shape, the heart, the driving force behind all our actions. The lulab, a palm branch, resembles the spine, which holds the body together and, without which, we would be unable to move. The hadas - myrtle branches - resemble, in their shape, the eyes, with which we behold God's world. And the arabot, the willow branches, resemble the lips, with which we give expression to our thoughts and feelings.
We take all of them together and we move them south, north, east and west, up and down, symbolizing our commitment in body, mind and hearth to follow God's commandments. God's presence, so to speak, is represented by signaling His Omnipresence in every corner of the world. (see more about the meaning of the four species here ).