They usually find it hard to communicate with others in a typical way and have difficulty understanding social conventions. As a result, individuals with autism may respond in unusual ways to everyday situations and changing environments.
Autism varies tremendously in severity. Individuals with severe autism conditions may have serious cognitive disability, sensory problems and symptoms of extremely repetitive and unusual behaviors. This can include tantrums, self-injury, defensiveness and aggression caused by an inability to communicate. Without appropriate intensive intervention, these symptoms may be very persistent and difficult to change. Living or working with a person with severe autism can be very challenging, requiring tremendous patience and understanding of the condition. In its mildest form, however, autism is more like a personality difference caused by difficulties in understanding social conventions.
Children with ASDs develop differently and at different rates from other children their age in the areas of motor, language, cognitive and social skills growth. They might be very good at advanced or complex skills such as solving math problems but find the "easy" things, like talking or making friends' very difficult. Some children with ASDs develop large vocabularies and can read long words but may be unable to vocalize the sound of a single letter. A child may also learn new skills, such as saying a number of words, but lose this ability later on.
The above information thankfully comes from the autismsocietycanada.ca at the following link.