Our sages learnt from the verse "And ye shall take you on the first day... and willows of the brook" (Vayikra 23, 40) that one of the four species is Arava - willow. We take 2 branches of willow and tie them on the left side of the Lulav

• What is a Willow - "A Willow has a red stem, the leaf is long and the edge of the leaf is smooth."
The Rishonim argue how to explain the Gemara which says that there is a Kosher kind of Willow which has a serrated edge. Rashi explains that all of the "teeth" face one direction and the Rambam explains that they are small. The Shulchan Aruch holds like the Rambam.
• What isn’t Arava - Populus isn’t an Arava although it grows next to water and so isn’t an isn’t almond. Although their leaves are similar, both are missing the Arava signs. Acacia saligna isn’t Arava, the leaves are a bit hard and it hasn’t got a red stem.
• Eucalyptus - the Eucalyptus leaves are long and the stem is red. Rabbi Tzvi Pesach Frank says in the name of Maharil Diskin that it’s Kosher as Arava, but most authorities prohibit it because it doesn't grow mostly on water. Eucalyptus is identified by its smell (and Arava according to the Midrash has no smell) and by the smooth edge while Willow leaves are a bit serrated.
• Types of Willow we shouldn’t use - Salix babylonica (Babylon willow or Weeping willow - the stem isn’t red and the "teeth" are sharp. White willow (Salix Alba) has a rough edge and it is better not to use it (This type is common!).
• Truncated - if the top of the Arava is truncated, it isn’t Kosher. A young bud is a clear sign that it is not truncated, though a young bud isn’t necessary.
• Stolen - it is very important to check that the Arava isn’t stolen (from someone's yard or a park), even when used only for the last days (when we can use lended 4 Minim).
• Length - The length of the Arava should be at least 3 Tefachim (24-30 cm).