What is autism and what should parents and caregivers know about recognising and supporting a child with autism?

I would like to understand what autism is, how it affects children, and what signs parents and caregivers should look out for in early childhood. At what age are the first signs of autism typically noticed and what do they look like in everyday behaviour? How is autism diagnosed and who should a parent speak to first if they are concerned about their child's development? What support, therapies, and resources are available for children with autism and their families? How can parents best support a child with autism at home and at school? Is autism more common in certain groups and is there anything known about its causes? I am looking for clear, compassionate, and practical information that any parent or caregiver can understand and act on.