pittom (stem):If this falls off you're in trouble!
surface:
A typical esrog has a very bumpy surface.
Its ideal color is mostly yellow,
or yellow and light green, as shown here.
uketz (tail):
If this falls off, don't worry about it!
From the Midrash:
There is an opinion held that the Tree of Knowledge from which Adam ate was an esrog tree. The Kabbalah concept is that our use of the esrog at Sukkot is a tikkun, or repairing, of Adam and Eve's act, by instead using the esrog for a mitzvoh.