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Here are a two different takes on the difference, from around the web!

1) Envy is coveting something (or someone) that somebody else has; jealousy is wanting something (or someone) that somebody else has but you think belongs to you. It's the feeling of being dispossessed.

http://www.englishforums.com/English/TheDifferenceBetweenEnvy-Jealousy/dnppk/post.htm

2) One difference between jealousy and envy involves the relationship between the jealous or envious person and his or her rival. An envious co-worker may develop a personal resentment towards a promoted co-worker because the position represents a higher salary and more responsibility. The true source of this envy is rarely the co-worker himself or herself, but the perceived value of the position. The co-worker may very well deserve the advancement because of his or her superior skills or education, but an envious person might become angry at himself or herself for not possessing those qualities.

Jealousy, on the other hand, focuses on the rival himself or herself, not necessarily the object or "good" at the center of the conflict. Jealousy implies a closer relationship between the jealous person and his or her rival. Instead of a promotion, the co-worker may start a romantic relationship with the jealous person's secret office crush. Because this rivalry is personal in nature, the target of the jealous person's resentment and anger is not necessarily the unattainable romantic partner, but the more attractive rival who now stands between them.

Jealousy is almost exclusively a negative emotion, while envy can has some positive effects, such as a renewed interest in self-improvement.

http://www.wisegeek.com/what-is-the-difference-between-jealousy-and-envy.htm