There is overlap and there is difference. Let's see if I can put it simply....
If you are more monogamous-inclined and not poly, then if you saw your partner talking to, flirting, or pursuing sex with another person then you would obviously get quite jealous.
If you are very poly-inclined non-jealous person, then that would not be an issue. However if you thought your partner was spending more and more time with his/her other partner and neglecting any good quality time with you, then you would understandably feel envious.
That make sense?