ADHD affects hundreds of millions of people. Here's what it is − and what it's not.

Though different people experience attention deficit hyperactivity disorder (ADHD) in different ways, it is something that can significantly disrupt a person's life. Whether in the classroom, workplace, or at home, ADHD has been shown to affect grades, job performance and can even cause relationship problems.

The Centers for Disease Control and Prevention (CDC) notes that ADHD is one of the most common neurodevelopmental disorders among children, but it's also known to be common among people 18 and older. 

It's a disorder that many have learned to live with every day, explains Wan-Ling Tseng, PhD, an assistant professor at Yale School of Medicine. They've done so through behavioral therapy or by adopting coping mechanisms.

In many cases, medication is also prescribed to improve focus and ease ADHD symptoms. "There are many effective, evidence-supported medical and non-medical treatments and other supports for ADHD across the lifespan," says J. Russell Ramsay, PhD, a licensed psychologist and retired professor of clinical psychology with the University of Pennsylvania's Perelman School of Medicine.

What is ADHD? 

ADHD is a chronic neurodevelopmental condition (referring to the way one's brain grows and develops) that "is characterized by difficulties with inattention, hyperactivity and impulsivity or both," explains Emma Hepburn, PhD, a clinical psychologist and author of "A Toolkit For Your Emotions." 

Though many people refer to ADHD and ADD (attention-deficit disorder) as separate conditions, they are mostly identical, so the term ADD is simply an outdated reference to ADHD, according to the Child Mind Institute. (One distinction between the two that some people point to is that some individuals with ADHD have hyperactive behaviors while others don't, but medically, the diagnosis is labeled ADHD either way.)

Symptoms of ADHD can "cause impairments in multiple life settings," Ramsay says.

How common is ADHD? 

Though it's considered more common in children, with at least 6.1 million kids in the U.S. between the ages of 2-17 living with the condition, per the CDC, it's also common in adulthood. "6.8% of adults around the world have ADHD," says Ramsay.

Indeed, childhood ADHD symptoms often persist into adulthood with such traits "usually consistent with one's age and development level," explains Tseng.

What are symptoms of ADHD? 

The symptoms of ADHD are pretty straightforward and include inattention - an inability to concentrate or focus; hyperactivity - frequent movements not fitting to the setting; and impulsivity - sudden actions that occur without thinking them through. 

These symptoms can manifest as a person seeming to daydream a lot, forgetting or losing things often, talking or interrupting a lot, having a tendency to squirm or fidget, or having a hard time resisting temptation, per the CDC. Tseng adds that a person with ADHD might also "have trouble following through on instructions, finishing tasks, or trouble with organizing thoughts," and/or "trouble waiting their turn."

It's also important to note: "Some people may experience more inattentive symptoms while others experience more hyperactive/impulsive symptoms," says Hepburn. She adds that such traits may also change over time as a person develops or circumstances change.