Wonderland, also known as Underland, is a mythical realm that was first located by Alice Liddell. Wonderland is accessed by an underground passage, and Alice reaches it by travelling down a rabbit hole. The world is located in Alice's mind and a creation of her vivid imagination. It is similar to a dream or a lucid dream, which is how Alice visits Wonderland, although it has been shown she is able to visit it while in a hypnotic daydream.
The land is heavily wooded and grows mushrooms. There are well-kept gardens and substantial houses, such as those of the Duchess and the White Rabbit. Wonderland has a seacoast, where the Mock Turtle lives. The land is nominally ruled by the Queen of Hearts, whose whimsical decrees of capital punishment are routinely nullified by the King of Hearts. There is at least one Duchess.
Despite this, Alice treats Wonderland and its inhabitants as if they were real living sentient beings, as shown when she cries and mourns the deaths of White Rabbit, Gryphon and Cheshire Cat. All three of them were killed and then revived. In Alice's next visit, Rabbit and the Mad Hatter were seen dying, only to be seen "alive" later. This is likely because death is treated very abstractly and metaphorically in Wonderland, and anything/anyone may return to "life" if it still has a place in Alice's subconsciousness.
During Alice's stays in Rutledge Asylum and the Houndsditch Home for Wayward Youth, the land underwent various changes and rulers and all of which were ended by Alice herself in her quest to regain her sanity. Wonderland transformed into a twisted demented nightmarish hell full of slavery, war, monsters, gore, corpses, animal experimentation and suffering which Alice couldn't escape from since Wonderland is part of her own mind.