A cultural interview is likely to be somewhat informal and comprised primarily of the people you'll be working closely with each day. They're trying to figure out if you're going to be a good fit for the office environment/culture - not your technical skills, but "will this person fit into the team well? How will they get along with the other individuals in the company who they'll have to interact with on a regular basis?"
In my group, we have a "team" interview after the initial round of interviews with the whole group where we ask a mix of technical and non-technical questions in a little more relaxed setting (as we're all peers or potential peers) to get a feel for how the candidate will get along. We know our department well, and also the people in other departments whom the candidate will be interacting with, and we can get a good idea about how those people outside the department will react to a new person.
Be yourself. If you put on a persona in the interview and get the job, you won't be able to keep that up daily for the next 5 years.