The autism spectrum, often called just autism or autism spectrum disorder, refers to a disorder of brain development caused by differences in the structure of the brain. Autism changes how someone thinks, understands the world, moves, communicates, and socializes. Autism is a spectrum disorder, which means that every person with autism will have different symptoms and different severity of symptoms. Some autistic people may need a lot of help with one thing. Other autistic people may not need help with the same thing. It may be easy to tell when someone is autistic, or it may not be.