There are a number of reasons you want to apply for Early Childhood Intervention Services:
1) The sooner you identify any delays in developmental milestones, the easier they are to correct
2) In some states (such as California) your ECIS case worker will be the one to help get you approved for Medicaid (MediCal in California)
Early Childhood Intervention Services is a product of the Federal Individuals with Disabilities Education Improvement Act (IDEA).
The program mandates a statewide, comprehensive, multidisciplinary service system to address the needs of infants and toddlers who are experiencing developmental delays or a diagnosed physical or mental condition with a high probability of an associated developmental disability in one or more of the following areas: cognitive development, physical development, language and speech development, psychosocial development, and self-help skills.
Once again, each State administers their Early Childhood Intervention Services differently. Some states may have it as part of their Early Head Start program, some may have it as different agency under a different name.
The goal of Early Childhood Intervention Services is to assist your child in meeting typical developmental milestones. For a more specific list of services, see Wikipedia's Early Childhood Intervention Services link.
Wikipedia's Early Childhood Intervention Services Link
Wikipedia's Early Head Start Link
In California, the Early Childhood Intervention Services program is called simply Early Start and is administered through the Department of Developmental Services. Services are provided through various "Regional Centers" and so the program is often referred to as "Regional Center", "Family Regional Center" or "FRC".
California's Early Start Link
How to Apply:
In most states (such as California), the way to access ECIS is by getting a referral from your pediatrician.