It's motion-capture 3D, so they basically film the actors with cameras all around them, and then their movements are mapped onto the skeleton of the 3D character. They used it in the film The Polar Express, as well as Monster House, although the technology was more basic then.
The problem with it is that sometimes, if the character is slightly off, because they look so much like a normal person, you instantly spot it and then you get the uncanny valley effect. I personally think that the Tintin movie does it extremely well, a reason that Steven Spielberg waited to make this movie was for better motion-capture technology to come out.