To put it simply, people with autism or autism spectrum disorders (ASDs) tend to have "impaired social interaction and communication, and restricted and repetitive behavior."
People with ASD can have have any number of many different possible characteristics. People like me with Aspergers, for example, usually have serious issues with socializing with other people, yet some of us have this only mildly and can live fairly normal lives while others have it much more seriously and have great difficulties coping. We may have sensitivities or desensitivities to various stimuli. I, for example, was very tolerant to physical pain as a child but as an adult have become overly sensitive to light, sound, and smell. I can't sleep at all if the smell of cooking enters my room. Many other people with Aspergers(aspies) will have different sensitivities or maybe none at all.
One thing that is easier to notice in people with ASDs is atypical eye contact. One may avoid eye contact all together or may stare too much, or even switch between the two depending on situations.