I'm trying to understand what is the difference in meaning between these two words: Зависть and ревность, I used both in meaning of "to be jealous" but I was told by someone that there is a difference between them (since the explanation was in Russian language I'm not sure that I understood it). What is this difference?
зависть – envy (a feeling of discontented or resentful longing aroused by someone else's possessions, qualities, or luck)
ревность – jealousy (affection-related, as if when a man is jealous of his wife or children of their parents etc.)
Both words are nouns. Note the spelling: it's "зависть", not "завист".
If you work from the English word 'jealous' it is true that both these words can be used, but only in accurate senses. See OALD definitions for 'jealous':
1) feeling angry or unhappy because somebody you like or love is showing interest in somebody else - In Russian this implies ревность.
2) jealous (of somebody/something) - feeling angry or unhappy because you wish you had something that somebody else has - In Russian it is usually understood as зависть.
So in these senses "to be jealous (of)" applies.
As for definitions of the Russian words:
Зависть usually means envy. And mostly used in this sense (sometimes in a positive way of course, or as a joke)
Ревность has at least 3 senses:
1) Jealousy in love, feelings, etc.
2) Being angry when someone is successful and you are not (e.g. at work)
3) obsolete meaning, zeal, huge desire, actions or energy to do or achieve something
To add to what was said above, the grammatical constructions for these two words are different.
завидовать (кому) For example, if someone just got a ticket to a movie you wanted to see you might hear: "Я завидую тебе белой завистью. - This is a "friendly" sort of envy, if such a thing exists. You're happy for the other person but don't secretly covet what they want.
-ревновать кого к кому-
Ревную свою девушку к её бывшему I'm jealous of my girlfriend over/because of her past boyfriend.