It was a group assessment, where we were given various assignments. First we broke into teams of two and were told to learn about the other, and were tested on the other person. Then, we were split in half and given more tasks to do. Team-building sort of stuff, and that took up the first half of the day, just getting to know one another.
The second half was understanding the company and our roles and the expectations of us once we entered (if offered the position). One on one interviews were then conducted, with the standard range of questions - what's your background, how do you think you'll fit in, why this company, etc.