A child may demonstrate a communication delay which falls in one of 
the following categories:  speech, language and/or hearing.  

Speech refers to the oral/vocal aspect of communication.  A child 
with delays in his speech demonstrates errors in his production of 
speech sounds.  Speech sounds are developmental in nature and are 
mastered as a child matures. Certain sounds do not require 
remediation until a child reaches a certain age.  For example, it is 
extremely common for a four-year old to substitute "w" for "r."  
He/she may say "wabbit" for "rabbit."

Language can be broken down into three components:  form, content and 
use.  Form refers to what is called syntax and morphology.  This 
includes a child's spoken grammar and sentence formulation.  Content 
includes receptive and expressive vocabulary knowledge.  Use is the 
way in which a child "uses" his/her language to achieve an end 
result.  This refers to the "social" aspect of language.  

There are numerous standardized tests which assess a child's speech 
and language development.  There are also websites that I have listed 
on the "links page" which outline speech and language developmental 
milestones.