ADHD Predominantly Hyperactive-Impulsive Type
The APA defines ADHD in these 3 parts because many ADHD children and adults have characteristics of both parts (ADHD combined type). But many may only have one part in isolation.
Coupled with inattention, this type presents itself in unlimited energy levels. The problem is that those energy can not be focused productively. ADHDers with hyperactivity as the predominant type, often do 101 things but completes (sometimes accomplishes) not that much. In adulthood this hyperactivity can change in hypo activity and even lethargy.
The impulsivity part of this type is often the most frustrating part of the two. Especially in adulthood, the other characteristics mellow down but impulsivity keeps on bogging ADHDers. It affects their social/personal and public lives (see ADHD Research results elsewhere on this site). The impulsivity causes ADHDers to react immediately on stimuli. They take a decision "on the spur of the moment" without considering all the facts, leading to misreading of misses important cues in the decision making process.
Impulsivity presents itself in 4 categories:
On emotional level, ADHDers jump to quick to conclusion about people and therefore have difficulty socializing. They misread important cues about peoples reactions resulting in a paranoia about these people's feelings towards them. This is also the category responsible for the often abusive nature of ADHDers, seeking for that adrenaline rush.
Intellectually, decisions are made quickly without contemplating the consequences and consider all the different options.
This is the ADHDers who are always risking their lives, not thinking that their actions are dangerous. "Nothing will happen", or "I didn't thought this would happen" are slogans that you might have heard.
This ADHDer verbalizes their mind without thinking what they said, resulting in often annoying and insensitive remarks towards people. There is a saying, "if you want the truth, ask an ADHDer" 1.
Please note that this was based on the definition of DSM-IV. DSM-V has been released and not much has changed - see Diagnostic Criteria here.