What it is, via NIMH:
All children with ASD demonstrate deficits in 1) social interaction, 2) verbal and nonverbal communication, and 3) repetitive behaviors or interests. In addition, they will often have unusual responses to sensory experiences, such as certain sounds or the way objects look. Each of these symptoms runs the gamut from mild to severe. They will present in each individual child differently. For instance, a child may have little trouble learning to read but exhibit extremely poor social interaction. Each child will display communication, social, and behavioral patterns that are individual but fit into the overall diagnosis of ASD.
Some Signs, From The Mayo Clinic:
Fails to respond to his or her name
Has poor eye contact
Appears not to hear you at times
Resists cuddling and holding
Appears unaware of others' feelings
Seems to prefer playing alone — retreats into his or her "own world"
Starts talking later than age 2, and has other developmental delays by 30 months
Loses previously acquired ability to say words or sentences
Doesn't make eye contact when making requests
Speaks with an abnormal tone or rhythm — may use a singsong voice or robot-like speech
Can't start a conversation or keep one going
May repeat words or phrases verbatim, but doesn't understand how to use them
Performs repetitive movements, such as rocking, spinning or hand-flapping
Develops specific routines or rituals
Becomes disturbed at the slightest change in routines or rituals
May be fascinated by parts of an object, such as the spinning wheels of a toy car
May be unusually sensitive to light, sound and touch and yet oblivious to pain
What many don't understand is that just because a child doesn't fit every descriptor, does not rule out autism. Yesterday on a public forum, I saw a mother say 'I think my son's diagnosis of autism is wrong, he makes great eye contact.' Another time, a parent lamented, 'But my son is so smart, there's no way it's autism.' Or, 'He loves other kids, the doctors are wrong about the diagnosis.'
These are only a snippet of the signs, and no child will meet every one. Autism is a COMPLEX disorder, the presence or absence of one (or even several) of the signs and symptoms doesn't rule out autism.
My son makes wonderful eye contact sometimes, he loves other kids most of the time, and he is super smart and VERY happy. But he's also autistic.
This video shows a lot of the signs as they present in one (adorable!) little boy:
If you want to read some more about the signs and symptoms of autism spectrum disorders, these are excellent resources:
Autism Web: Autism Symptoms and Warning Signs
CDC: Signs and Symptoms of ASD's