ADHD is an acronym for Attention Deficit/Hyperactivity Disorder. People with ADD and ADHD struggle to maintain focus on certain tasks as they have a more difficult time "tuning out" extra stimuli. They can become overly attentive to distractions, and/or be impulsive and over-active. Many children are "wiggly" and inattentive, but these children have these conditions outside the normal range for that child's particular age.
Questions that are tagged with ADHD should be about coping strategies for parenting and educating these children, and the special situations that occur because of their diagnosis.
Based on the types of symptoms, three kinds (presentations) of ADHD can occur:
Combined Presentation: if enough symptoms of both criteria inattention and hyperactivity-impulsivity were present
Predominantly Inattentive Presentation: if enough symptoms of inattention, but not hyperactivity-impulsivity, were present
Predominantly Hyperactive-Impulsive Presentation: if enough symptoms of hyperactivity-impulsivity but not inattention were present
Because symptoms can change over time, the presentation may change over time as well.
Quoted from DSM-5, http://www.cdc.gov/ncbddd/adhd/diagnosis.html
Many children diagnosed with ADHD have a very difficult time sitting still or participating in the average classroom setting without incurring behavioral consequences for movements over which they have little control. They also may have a difficult time focusing well enough to complete common tasks given at school. For these reasons, children with ADHD are often medicated to help decrease incidence of overactivity and increase focus and attention span. About 3-5% of today's children have been diagnosed with this disorder making it the most common behavioral disorder affecting children today.
In the most severe cases ADHD is often coupled with other disorders such as Tourette's or Bi-polar disorders. ADHD sypmptoms can overlap with these disorders as well as depression, anxiety and others so diagnosis must be made carefully.
For a more detailed list of symptoms, synopses of types of treatments, and listing of tests that are administered to determine a diagnosis of ADHD or not, check out these more in-depth links.