Jealousy is a perceived thread of loss, of a relationship, whereas envy is desiring something that someone else has. The big difference is that jealousy is often driven by insecurity. It's often driven by some narcissistic elements, whereas, envy is often based on unfairness. Somebody has something I want.
I think, in English language, the word jealousy, or I'm jealous, is often used incorrectly. People say, 'Oh, I'm so jealous that you have this great new job.' In reality, the correct word would be envious. You're envious that someone has something you'd like to have. You're jealous, if you feel that there is a threat to something that you like, and you'd like to keep, so, you would feel jealous if your husband, or boyfriend, is talking to another woman for a prolonged period of time, and seems to be interested in her.
Normal jealousy is correlated with relationship investment. That is, if you have a lot invested in the relationship, if you really want to keep this relationship, you're more likely to get jealous. You're more likely to want to make sure that there are no threats to this relationship.
Pathological jealousy, on the other hand, is suspicious type of jealousy. It's not based on facts. It's based on your thoughts, on suspicions. That type of jealousy is very destructive to the relationship. It consists of a lot of nagging, interrogations, whining kind of behavior, that usually annoys your partner.