A person who is very competitive, impatient, hostile, or just stressed is often said to have a type A personality. These people are often also highly organized workaholics, who may be very successful at their jobs, but the traits that most identify type A personalities are high stress levels, impatience, and often hostility. It is often hypothesized that type A's have low self-esteem, but since most Americans have low self-esteem, we should be careful not to confuse correlation with causation.
People with this personality type are more likely than others to have problems with hypertension and heart disease. This is likely caused by the feelings of hostility and stress found in many type A personalities, and not something that will be a problem to 'happy' type A's. Many modern psychologists consider the type A/B classification to be outdated and not particularly useful.
Type A personalities are often contrasted to Type B personalities, people who are relaxed and easy-going. Since obviously there are many people who don't fit neatly and obviously into either category, some people speak of a Type AB personality. There is also a Type T personality (risk taking behavior), and the Myers-Briggs Type Indicator, for those who want more detail and finesse.