ADHD is a neurodevelopmental difference. ADHD is characterised by difficulties and difficulties with executive functioning.

Executive functioning is a set of mental skills that include working memory, flexible thinking, and self-control. These skills allow us to plan, focus attention, remember, and juggle multiple tasks.
ADHD can present as:
Predominantly inattentive
This is when the child or young person presents primarily with differences with focus, attention and concentration.
This was previously known as "ADD".
This can look like:
Predominantly hyperactive/impulsive
This is when the child or young person presents primarily with hyperactive and impulsive behaviours.
This can look like:
Combined presentation
The most common presentation.
This is when a person has both the inattentive traits and the hyperactive/impulsive traits.
Most people with ADHD have traits from both the inattentive and hyperactive/impulsive sides.
ADHD is NOT