Attention-deficit/hyperactivity disorder (ADHD) is a mental disorder that affects a person's ability to regulate their attention and behavior. There are three main types of ADHD: inattentive type, hyperactive-impulsive type, and combined type.

1. Inattentive type ADHD is characterized by difficulty paying attention, difficulty following instructions, and being easily distracted. People with this type of ADHD may have trouble completing tasks and may be forgetful.
2. Hyperactive-impulsive type ADHD is characterized by excessive activity and difficulty controlling impulses. People with this type of ADHD may fidget constantly, talk excessively, and have trouble waiting their turn.
3. Combined type ADHD is a combination of the inattentive and hyperactive-impulsive types. It is the most common type of ADHD.

The exact cause of ADHD is not known, but it is thought to be related to a combination of genetic and environmental factors. Some research suggests that certain genes may make a person more likely to develop ADHD, and that certain environmental factors, such as alcohol or drug use during pregnancy or exposure to toxins, may increase the risk of ADHD.

It's important to note that ADHD is not caused by bad parenting or a lack of discipline. It is a medical condition that requires appropriate treatment.