A memoir is different from an autobiography. Memoirs are often like novels: they've got scenes, dialogue, fancy language with lots of similes and metaphors. An autobiography, on the other hand, is a little more like a dusty history textbook. Another major difference is that an autobiography is often considered to be more or less historically accurate: there are sources included for every major detail. What's the source for a memoir? The author's own memory, that's what.
For all we know, Jeannette Walls made everything up in The Glass Castle. That's unlikely, because she would have been called out by now if she had. But memoirists are free to fudge details like dialogue and character names. Do you think Jeannette remembers everything her mother said, word for word? We doubt it, but she definitely recalls the gist of it.
Memoirs are often coming-of-age stories, and The Glass Castle is no different. The vast majority of the book deals with Jeannette's childhood and adolescence, from when she is three until she is in college. After that, she glosses over her adulthood. This book is about her formative experiences, with very little on the adult she became.