By convention a Lemma is a technical intermediate step which has no standing as an independent result. Lemmas are only used to chop big proofs into handy pieces. (Quoted from here)
I am wondering why Zorn's Lemma is called a lemma at all. Is it a "handy piece" of some proof of an important result?
On
From Proofs from the Book, Second Edition, Martin Aigner and Günter* M. Ziegler, p. 149:
The essence of mathematics is proving theorems -- and so, that is what mathematicians do: they prove theorems. But to tell the truth, what they really want to prove, once in their lifetime, is a Lemma, like the one by Fatou in analysis, the Lemma of Gauss in number theory, or the Burnside-Frobenius Lemma in combinatorics.
Now what makes a mathematica statement a true Lemma? First, it should be applicable to a wide variety of instances, even seemingly unrelated problems. Secondly, the statement should, once you have seen it, be completely obvious. The reaction of the reader might well be one of faint envy: Why haven't I noticed this before? And thirdly, on an esthetic level, the Lemma -- including it's proof -- should be beautiful!
Edit: I was so excited to drop that quote that I didn't carefully read the question. Here's a link to Timothy Gowers's Weblog where he describes "How to use Zorn’s lemma" which hopefully more directly answers the question, including:
Quick description: If you are building a mathematical object in stages and find that (i) you have not finished even after infinitely many stages, and (ii) there seems to be nothing to stop you continuing to build, then Zorn’s lemma may well be able to help you.
*Cut-and-pasted the "ü" in Günter per suggestion in the comment -- LaTeX encoding didn't seem to work.
When you want to construct something very large, or just handle something very large, you can do it using the axiom of choice and/or the well-ordering theorem coupled with transfinite recursion and induction.
It's not very difficult, but one has to understand the structure of the ordinals in order to use the ordinals properly. In particular one has to know a bit more about well-orders in order to use transfinite induction.
If someone wants to prove that every vector space has a basis; or if every unital commutative ring has a maximal ideal; or whatever. They might not be very interested in well-orders, ordinals and transfinite recursions.
Zorn's lemma offers a way out. It offers working with partial orders, often we use subsets or some other objects that we know sufficiently well.
And of course, it has plenty of easy uses, since partial orders where every chain has an upper bound are abundant in mathematics (see two easy examples above). It is a lemma since we're not generally interested in the partial order or in finding a maximal element there. We are, however, interested in specific partial orders (the set of linearly independent sets ordered by inclusion, for example). So the term "lemma" is very fitting, since it tells you some nontrivial information about a general context that is applicable to your case.