Culture and background are separate options. You pick your culture first, which determines where you come from, and your background establishes what your job was. The jobs list is the same no matter what culture you were from (leading to a few odd possible combinations, like if you're from one of the low-tech barbarian cultures yet have an aristocrat background).
The culture option has a pretty decent-sized impact on a lot of things. It affects your starting gear, adds a point or two to specific attributes, and opens up a bunch of conversation options. As far as dialogue options go, culture has more of an affect than race by a long shot, at least so far. In most standard fantasy RPGs your race usually unlocks a bunch of conversation options, but in PoE's world your culture is more important; there really isn't much fantasy racism in its world. There is a lot of prejudice based on other things, though, don't worry. In fact, the only real race-based dialogue and interaction options I've seen are for godlike and that's it.
Background is finer-grained and establishes what role your PC had in whatever culture s/he hails from. It opens up a few dialogue options and gives you a bonus to one or two skills. It has less of an impact on dialogue options than culture, at least so far.