Autism and Autism Spectrum Disorder (ASD) are both general terms for a group of complex disorders of brain development. They include:
Autistic disorder - “classic autism”
childhood disintegrative disorder,
pervasive developmental disorder-not otherwise specified (PDD-NOS)
Those affected by Autism & ASD show the following symptoms/characteristics (in varying degrees):
Difficulties in social interaction
Difficulties in verbal and nonverbal communication
difficulties in motor coordination
Attention and physical health issues like sleep and gastrointestinal problems.
Autism has its roots in early childhood brain development. Signs of autism and symptoms of autism tend to appear between 12 and 18 months of age. Some children begin to develop normally until they are 2 years old, but after that they lose skills and develop autism - “regression"
Source: AutismSpeaks, Wikipedia