Everyone experiences jealousy at some point.

Jealousy is characterized by

• Suspicion or anger about betrayal
• Distrust
• Low self-esteem and sadness over loss
• Uncertainty and loneliness
• Fear of losing an important person to an other

Jealousy lies somewhere in the gray area between sanity and madness. Some jealous reactions are so natural that a person who doesn't show them seems in some way "not normal." Others seem so excessive that one doesn't need to be an expert to know that they are pathological.

Types of jealousy include:

Romantic jealousy:This is probably the most frequently experienced type of normal jealousy. A 2004 study published in Evolutionary Psychology found that, when asked about sexual versus emotional infidelity, respondents of both sexes reported more jealousy about their partner’s emotional infidelity.

Work or Power jealousy: This strikes people who are jealous about a "missed" promotion, salary level or other work-related issue.

Friend jealousy: People, especially adolescents, are often fearful of "losing" a friend to an interloper.

Family jealousy: Sibling rivalry is one trademark characteristic of this type of jealousy.

Pathological jealousy: Any of the above can achieve an extreme form and become all cosuming, interfering with daily activities and work and causing disrtress to self or others.  

More about Pathological Jealousy

Pathological jealousy results in delusions of unfaithfulness, causing a jealous reaction, where a person perceives a threat to the relationship when there's no threat present. Only an expert can determine when "normal" jealousy  changes into pathological jealousy,

"Abnormal" or pathological jealousy is usually associated with the exercise of excessive control
over the partner with unrealistic concerns about the partner's faithfulness.

People with pathological jealousy jealousy often exhibit behaviors such as repeated calling, checking the phone, diary, etc., stalking, cyberstalking, confinement, or isolation of the partner, sabatoge of the partner's relationships,  or even violence.

A classic example is the man who is so suspicious of his loving and faithful wife that he constantly spies on her, listens in on her phone conversations, or the woman who records the mileage in her husband's car for unexplained trips--and despite  repeatedly proven fidelity continues to suspect the spouse and suffers from tremendous jealousy.

Here, therefore, it is important to differentiate "normal" from "delusional" jealousy. Normal jealousy has its basis in a real threat to the relationship; Jealousy is indeed the shadow of love. In the example above, woman may have fallen in love with her husband because he made her the center of his world. Her husband, however, may have fallen in love with her because she was beautiful--the kind of woman he only dared dream about as a shy adolescent. His jealousy, now, is focused on his feelings of inferiority and insecurity and fear of losing her. On the other hand, if the husband is suffering from delusional jealousy, any sign – his jealousy may be based on interpretation of  a meaningless spot on clothing, or a short delay in arriving home -- as evidence that his wife is being unfaithful.