They're separate.  In the 2014-2015 TV season, there's **a ton** of DC shows:

* *Arrow*
* *The Flash*
* *Constantine*
* *Gotham*
* *iZombie*

*Arrow* and *The Flash* are the only shared universe amongst these.  At [Comic Con 2014](http://io9.com/ras-al-ghul-comes-to-arrow-and-other-dc-tv-news-from-co-1611625466) it was stated that there's no shared universe amongst the others, as of yet:

> While we know that *Arrow* and *The Flash* will crossover a lot, it was deemed a little difficult to cross over the non-CW shows, since *Gotham*'s time period is a little uncertain. *Gotham* executive producer Bruno Heller said, "Never say never" but pointed out "If those two [*Gotham* and *Constantine*] worlds coincided, then it would work. But I'm not really sure of the chronology. Maybe *Constantine* is only five when *Gotham* takes place."

--------

Following the cancellation of *Constantine*, Constantine then guest starred on an episode of *Arrow* (S4E5 - *Haunted*).  This then retroactively linked *Constantine* to the *Flash* and *Arrow* universe.