The dogs just behind the lead dogs are called the swing dog. They are often leaders in their own right. Just in front of the sled are the wheel dogs. They are very important in helping to guide the sled through turns. Wheel dogs have as much responsibility as leaders in that they must keep the sled from hitting trees or boulders when making turns or just to bring the sled around without tipping it over.
Wheel dogs are usually a little larger or more muscular than the other dogs on the team.
The other dogs are team dogs, although most of them can handle swing or wheel or even lead when needed. By the way, there is no special breed of dog used on the Iditarod. Over the years, mushers have mixed all sorts of breeds in attempts to find the perfect sled dog. Some have been quite successful. Once underway, the team will run virtually on autopilot, almost always choosing the correct trail.