One of the enemies you'll face in "Modern Warfare 2"'s Spec-Ops mode is aptly named The Juggernaut. Basically imagine a huge, hulking dude in a bomb defusal suit who will march at you until you manage to take him down with explosives and .50 caliber rounds. It's the equivalent of a boss character in Spec-Ops and you really don't want to get on the business end of one of these guys.
The lead character designer for "Modern Warfare 2" discusses the creation of the Juggernaut in the video up top. Enjoy!