And thou shall take for yourselves on the First Day (of the Succoth Holiday) fruit of a beautiful tree, date palm branches and twigs of a plaited tree and willows of the river (Leviticus 23:40)

The Four species are identified as follows:

Fruit of a beautiful tree - Esrog (Citron)

Date Palm Branches - Lulav (Palm)

Twigs of a plaited tree - Hadasim (myrtle branches). Tradition follows the opinion that requires 3 branches.

Willows of the river - Aravos (willow branches). Tradition follows the opinion that requires 2 branches.

1. The 4 Species all taken together make up one single Mitzvah. If any of the species are missing then you have not fulfilled the Mitzvah at all.
One Lulav,
One Esrog,
Two Aravot
Three Hadassim
are taken on Sukkot. The Lulav, Hadassim and Aravot are bound together.

The four species are not taken on Shabbat even if Shabbat is the first day of Sukkot. The Mitzvah does not apply at night.

2. Performance of the Mitzvah on the first day of Succoth requires legal ownership of the species. A borrowed set is not fit for the mitzvah. On the remaining six days ownership is not strictly required.

3. Beauty. Any one of the 4 species that lacks the Jewish legal standard of beauty is invalid. A dried out member of the 4 species would invalidate the mitzvah for this reason. This has to be done for the sake of the Mitzvah; not for the purpose of showing off to neighbors.