Ya’akov’s tefilah at the opening of VaYeitzei marks the institution of tefilas arvis. The gemara (Brachos 27) writes that tefilas arvis reshus, as opposed to shacharis and mincha, which are real chiyuvim. The gemara does not mean that one can arbitrarily skip ma’ariv – what the gemara means is that if a person is engaged in some other mitzvah, that other mitzvah takes precedence over ma’ariv (Tosfos; see R’ Chaim al HaRambam, Hil Tefilah, for a different approach). Many poskim write that since we now all customarily daven ma’ariv, it is no longer treated as reshus, but has the same status as other tefilos.
Why is ma’ariv considered a tefilas reshus? Some suggest that Ya’akov was caught offguard by the sudden setting of the sun. His tefilah was unplanned (he may have in fact been intending to daven mincha – see Divrei Shaul of R’ Yosef Shaul Nathanson) and not formalized in the same was as shacharis and mincha.
Since ma’ariv is reshus, poskim write that even women who daven shacharis and mincha may skip it. Although men have accepted upon themselves to treat ma'ariv as obligatory, women have not. (Howoever, it’s worth noting that Rabeinu Yonah in his Sha’arei Teshuvah does make reference to women davening three tefilos a day.)
An interesting exception to the rule: the Sha’arei Teshuvah (on Shulchan Aruch, citing Mor u’Ketziya) writes that ma’ariv on Shabbos and Yom Tov is not a reshus, but is a chiyuv. The logic is that ma’ariv on Shabbos and Yom Tov serves a dual-function: it is a kiyum mitzvah of tefilah, but it is also a kiyum mitzvah of being mekadesh Shabbos or Y”T (R’ Yonasan Shteif). It follows that women must also daven ma'ariv on these nights.
Practically speaking, this helps resolves another halachic difficulty. The Magen Avraham writes that a man is yotzei his mitzvah of kiddush d’oraysa when he davens ma’ariv. Asks the Dagul m’Revava: if so, how can he say kiddush for his wife and be motzi her? Her chiyuv kiddush is d’oraysa, his is only derabbanan – you can only be motzi someone if both parties share the same level of chiyuv? If one’s wife or daughter also davens ma’ariv, fulfilling her d’oraysa of kiddush, this question is moot.