Let's be really clear about this – there are two titles here, since there are two books. The first is Alice's Adventures in Wonderland, which is exactly what it sounds like – a series of wacky exploits by our heroine, Alice, in a strange fantasy realm known as Wonderland. Describing the book as "Adventures" is telling, since it clues us in to the fact that the plot is episodic (lots of little adventures, often one for each chapter) instead of one long story arc (like most novels). It's also important that the title tells us the imaginary world in the book is called "Wonderland," since Alice never learns the name of this strange place during her adventures. In fact, the only time that the word "Wonderland" occurs in the book is in the second-to-last paragraph of the last chapter, when Alice's sister reflects upon Alice's adventures. So the title of the book also clues us in to the fact that we are focused on Alice, but that we as readers know things she doesn't about her experiences.
The title of the second book is Through the Looking-Glass, or, if you want to get really specific (and we know you do!), Through the Looking-Glass and What Alice Found There. A looking-glass is a mirror – you know, because a mirror is a piece of glass with a foil backing in which you can look at yourself. In a literal sense, of course, there's nothing "through the looking-glass," because it has no other side; but in another way, the other side of the looking-glass is a reflected world, a place that's the backwards or opposite version of the "real" world. So this title indicates that the book will constantly play with the relationship between something and nothing, fantasy and reality.
We're going to call these stories "the Alice books," just to give ourselves an easy short way to refer to them. You may also have heard one or both of the books referred to as Alice in Wonderland, which is the title given to many of the film adaptations of the stories. It's important to realize that (a) this is a different version of the title than the one Carroll gave his book and (b) taking out the word "Adventures" removes some of the episodic feel, suggesting a more unified plot and organization.
There's one more title that lurks behind the Alice books, the title of a book that never got published because it turned into Alice's Adventures in Wonderland. We're talking about Alice's Adventures Underground, which was the title that Lewis Carroll gave to the first draft of his manuscript. It's interesting to think about this change, since Carroll seems to have decided that he wanted Alice to journey through a world that was totally fantastic, a "Wonderland," instead of a world that had a literal relationship to reality. You can even view pages from the manuscript of Alice's Adventures Underground online – check out our "Best of the Web" section.