Jealousy is that unpleasant emotion you feel when you think someone’s trying to take what’s yours. Jealousy might creep up on you if someone flirts with your sweetie or copies your unique style.
Jealousy is called the "green-eyed monster" because it can make people obsessed with how they compare to others, always trying to beat their rivals by calling attention to what they have and seeing if the rivals have any reaction. Don't confuse jealousy with envy, which can sometimes be a slightly more positive emotion. If you envy your friend's athleticism, you wish you had it too, but you also admire her dedication.