From my booklet "Experience the High Holidays" with Lubavitch Youth Organization:
The Four Species are:
The Etrog - (citron) - which tastes and smells wonderful. This symbolizes the achiever, a person of great knowledge and Torah scholarship.
The Aravot - (willow branches) - has neither taste nor smell. This symbolizes the nondescript - the person who lives the plainest life.
The Lulav - (palm branch) - This comes from a date-palm tree and has great taste but no smell. This symbolizes a committed scholar with knowledge but little "sweat equity" in mitzvoth.
The Hadasim - (myrtle twigs) - have a good smell but no taste. This symbolizes the doer, a person with a lot of mitzvoth but no serious learning.
A sukkah is enclosed, and the covering on the sukkah – are branches, twigs – and when it rains, it rains in. It is essentially unprotected on top because in the sukkah we express prayer and the Thanksgiving for G-d’s protection of our ancestors following our exodus from Egypt.
Wishing all our Jewish friends and family a Happy Sukkot holiday!