Autism Spectrum Disorder (ASD) is classified by the CDC as "a developmental disorder caused by differences in the brain." Sometimes, these differences are known - such as a genetic condition. Other times, however, it is unknown. Even though scientists believe there are multiple aspects of a persons life that all act together to change common development, there are still a lot of unknowns about ASD.
People living with ASD often behave, learn, communicate, and interact with others in ways that are different from most people. There is usually nothing about the way someone looks that can tell you whether or not they have autism. It can have a variety of effect on people diagnosed - for example, some folks are completely non-verbal while others communicate just fine, and some people with ASD need a lot of help in their daily lives, while others do not need the same assistances. This is because Autism isn't a cut and dry, black and white type of diagnosis - it is a scale, and a spectrum.
Signs of Autism usually appear before the age of 3, and will last throughout a persons life - although some symptoms may improve over time. Children with autism growing into teenagers, and into adulthood, often find themselves struggling with communicating with their peers, developing friendships, and understanding social queues for behaviors expected at work or school. This can go hand-in-hand with other mental health issues, such as anxiety, depression, and ADD/ADHD (Attention Deficit/Deficit Hyperactivity Disorder) which are all more common in those also living with Autism.
But what are the signs?
There are a lot of behaviors that can categorize Autism, but also categorize a wide variety of other disorders and mental health situations. Because of this, it can be complicated to diagnose with 100% certainty - especially if the Autism symptoms are the same as other symptoms, such as aspects common with ADHD.
Some common signs of autistic behavior:
Does not like/use eye contact, or avoids it all together
Unable to notice when someone is hurt or upset around them
Incapable of picking up social queues, seems to be living in their own little world (not antisocial, just missing the queues)
Factual, instead of imaginative (this is not to say that people with ASD are not extremely creative)
Repetitive/Restricted behaviors and interests (obsessive routines, repeats words or phrases, flips hands or rocks body in a physical 'stim', gets very upset with minor change)