Envy and jealousy are frequently used as synonyms in conversational English. They are, in some usages, remarkably close in meaning, but distinguishable.
Jealous derives from zealous, which derives from the Late Latin "zelus" which arrived on England's shores via the Old French "gelos" and became the Middle English "jelous."
The connection between jealousy and zeal is most apparent in the Greek zeloo and zeleuo and zelos are variously translated as either "zealous" or "jealous." These terms are used in both positive and negative senses, e.g. sometimes it is a good thing to be zealous for God or for justice or for peace but a bad thing to harbour feelings of jealousy. The Authorized (or King James) Version of the Bible makes this difficult in that it sometimes describes God as jealous -- that is, filled with zeal for His people.
Envy derives from the Latin invidia which, via the Old French, became "envie" in Middle English.
Too many distinctions without support are offered in too many authoritative prescriptive sources. An attempt to follow them all would lead on to eschew the use of either word ever.
By far the simplest and best explanation I can offer treats these terms as moral categories [so what did you expect from somebody who teaches ethics and moral theology?].
I would suggest that jealousy is about what is mine and envy is about what is yours.
To be jealous is to fear that you have designs on my possessions, or on things that I fancy as rightfully mine, or that are now in your possession but were formerly mine, or that I thought ought to be mine.
To envy you is to experience resentment and discontent arising from your achievements, advantages or possessions. Some wag once said that "envy is the art of counting the other fellow's blessings."
For more than you ever wanted to know about the distinction, see Saint Cyprian at http://listserv.american.edu/catholic/church/fathers/cyprian/cyp-jeal.txt
