While everyone with an autism spectrum disorder has a difficult time with social skills and communication, people with severe autism are most likely to be entirely unable to use spoken language. They may also appear to take no notice of the people around them. It's important to know, however, that appearances can be deceiving: some people with severe autism have learned to communicate using sign, spelling boards or other tools, and they make it clear that people with severe autism may well be much more aware than they appear to be.
Many people on the autism spectrum have sensory dysfunction (they're too sensitive or not sensitive enough to light, sound, touch, taste or smell). People with severe autism tend to be extremely sensitive, such that going out into crowds, bright lights or loud noises can be overwhelming.
Most people on the autism spectrum have repetitive behaviors and self-stimulatory behaviors. People with severe autism are likely to have many such behaviors, and those behaviors can include self-injury (head banging, hair pulling, etc.).
People with severe autism may also have physical symptoms that sometimes appear with less profound autism. These may include sleeplessness, epilepsy, mood disorders, and, according to some sources, gastrointestinal issues.
Treatments for severe autism usually include Applied Behavior Analysis along with speech, occupational therapy, physical therapy and, sometimes, play therapy. Parents should be sure that their doctor checks their child with severe autism for any physical problems which could be contributing to difficult behaviors.