Workaholic - One who has a compulsive and unrelenting need to work.

One of the main differences between a motivated worker and a workaholic is perspective. A well-adjusted worker realizes that his job is only one element of his identity. Once the work is done, the workplace is replaced with a fulfilling home life.
For a workaholic, on the other hand, her job title becomes a much larger percentage of her identification. There is no such thing as 'leaving the workplace' - there is work and there is a very long break. A true workaholic usually has difficulty dividing work life from home life.
Some people may consider someone who works extremely long hours to be a workaholic, but that has not proven to be entirely true. Certain occupations require a significant time commitment, such as the medical or legal professions, but a workaholic is not defined by the raw number of hours spent at work. A workaholic is more likely to volunteer for overtime hours or agree to take on a large project late in the week. Salaried employees may feel an obligation to work long hours, but a workaholic often feels extremely anxious or depressed if he does not work until exhausted.
One self-test to determine whether you are a true workaholic is to observe your behavior outside of work. Make a mental note of how many work-related conversations you start with other people. Do you insist on discussing the poor performances of your co-workers or employer? Is your sleeping disrupted by work-related thoughts? Do you find yourself driving by your workplace during off-hours? If so, you may very well be a workaholic, or at least a workaholic in training.
Unlike an active alcoholic or substance abuser, a workaholic is not automatically penalized for pursuing his or her addiction on the job. A workaholic's addiction is the job. Company policy may prohibit alcohol or drugs on the premises, but it doesn't always address performing too much work. Counseling a workaholic often requires sensitivity on the part of the employer, since losing such a motivated employee could be detrimental. A true workaholic may need to seek professional counseling in order to determine the root causes of her compulsion to work.