In order to be eligible for special education under IDEA, the child has to meet the criteria for eligibility in one of the eligibility categories in the law. These include: serious emotional disturbance, learning disabilities, retardation, traumatic brain injury, autism, vision and hearing impairments, physical disabilities, and other health impairments. If the child meets the criteria of one or more of these categories, requires special education or related services, and his or her disability adversely affects educational performance, the child may be eligible to receive services.
Although ADHD is not one of the categories for service, children can receive special education or related services under the category "other health impaired." If your child has been diagnosed with dyslexia, he would probably be given services under the "learning disabled" category. Once the child is classified, an Individual Education Plan (IEP) is generated that lists goals and objectives for that child as well as a way of tracking progress within the designated program. Parent input is critical during the IEP process.
Many children who do not meet the criteria for IDEA may receive accommodations under another law, Section 504 of the Rehabilitation Act of 1973. That law requires public-school districts to provide a free, appropriate public education to every "qualified handicapped person" residing within their jurisdiction. If the child is found to have impairments that substantially limit a major life activity (in this case, learning) the education agency has to decide what related aids or services are required to provide equal access to the learning environment. The law states that the child's education must be provided in the regular classroom with supplementary aids and services if at all possible. The school or school district generates a Section 504 Accommodation Plan every year that outlines the child's needs and exactly what accommodations/modifications are requested.
An excellent, comprehensive resource you might want to look at is Lawrence M. Siegel's The Complete IEP Guide: How to Advocate for Your Special Ed Child. Siegel walks parents through the entire IEP process and provides excellent resources for more information about the legal safeguards for children with special needs.