Do you have a child who is 2 or 3 years old and is not talking, often not performing tasks that you demand from him, and yet you think he is understanding your speech? Does he have first class tantrums? Does he get frustrated because he expects you to understand what he wants without him saying it, and sometimes you don't? Does he go to school and seem to be retarded compared to his peers, not talking and not participating in activities? Is he exceptionally good at solving puzzles, concentrates on tasks he chooses?
Read this book and you might see him described here. He might also be autistic, or have Asperger syndrome, read about those too, but be cautious with early intervention, do the early intervention yourself, spend more time with him, put him to sleep with you in your bed, talk to him, take him on trips and walks, avoid routine, these things can't hurt. I read this book when my child was 2 and not saying one word, not even mama. Now he is 2 and a half and just started talking.