What is “Through the Looking Glass” for the Alice in Wonderland?

I mean I’m just really confused and it is kind of driving me insane how I see “Alice in Wonderland and Through the Looking-Glass” and I’m really confused about the Through the Looking-Glass part… Can anyone clarify? Is it in the Disney movie? What is it? And one more thing, I heard about a lot of other sequals and related stories to Alice in Wonderland? Can anyone explain what the other stories are?