What happens when Coraline goes into the door?

Neil Gaiman's Coraline tells the story of our protagonist, the titular Coraline, who lives with her parents on the second floor of an older home. While exploring the intricacies of her new home, Coraline comes across a locked door, which her mother opens to reveal a brick wall. Bored and stuck at home, Coraline explores the door again on her own and instead of a brick wall, finds a hallway.
After traveling through the door, Coraline is surprised to find an apartment that looks like her own, but with small differences. She hears her mother calling her name, and when she finds the source of the noise, it's a woman who looks a lot like her mother, except she has black buttons for eyes, and long, pointy fingernails. Coraline soon learns that this woman is her "other mother" and that she also has an "other father."
In addition to her "other" parents, there are "other" versions of the neighbors she knows from her real home. She runs into her neighbor's cat who informs her he is not an "other" cat, because cats "keep ourselves together."