Merriam-Webster defines territorial, spot-on: "...defending an area against intruders, especially of the same species." Conversely, jealousy equates to being suspicious, wanting someone else's achievements, and being fiercely protective.
There's this idea that a little bit of jealousy is a good thing. The good thing actually being referred to is territorial. No amount of jealousy is a good thing. It breeds insecurities, accusations, and mistrust. If you're with someone who generates a jealous feeling within you, it may be time to evaluate the level of trust that may be affecting the lifeline of your relationship.
Jealousy is an ugly trait, but it is fixable. It usually stems from a history of not having (i.e. better jobs, clothes, relationships, money, etc.) Without an attitude of gratitude for the things you already have obtained you will naturally form an obsession, or become fixated, with the greener grass. When things seem better than what you have you will form feelings of frustration that point to you not truly being happy with yourself, overall. It is no one's responsibility to act rather than react in this situation. It is also important to pick partners that do not encourage trust issues or envious thoughts (i.e. flirtatious with women where you will be able to see, complimenting other women in front of you, making you feel less than so when you see more than you long for it, etc.). Once you build a strong enough fortress around your emotions to not fall apart at everything that could indicate imperfection you will become less focused on trying to be what you think others need you to be, and draw more attention to having confidence in the face of difference.