Developmental Delay

I would like to know What is in reality a developmental delay.  After pushing my son's pedriatrician to send me to see a neurology; I found out that my son has a developmental delay.  He is now 22 months and he just started to walk.  He is walking like he walked before.  He says few words but not alot.  I am very worry because when I talk to him he won't pay that much attention.  As an example when I asked him where is your sister he won't even looked at her. Even though he knows how to say daddy he is seing confuse. Like  when  I go to the babby sitter and picked him up.  I would like to if he will grow up of this later.  He is in speech therapy, outpatience therapy and physical therapy.  PLease let me know asap.