Story Line

The player assumes the role of various characters during the single-player campaign, changing perspectives throughout the story. The playable characters are special forces operatives conducting black operations behind enemy lines. In this way, the player's characters have their own traits such as voices and shadows. Each mission features a series of objectives that are displayed on the heads-up display, which marks the direction and distance towards and from such objectives. The player is accompanied by friendly troops throughout the game. Although primarily a first-person shooter, the player pilots a Hind helicopter and guides friendly troops from a SR-71 Blackbird reconnaissance aircraft. The campaign features several scripted cinematic moments. One of them is a bullet time effect during the "Victor Charlie" level, activated when the player fires toward the last enemy of a Viet Cong squad.

Source: http://en.wikipedia.org/wiki/Call_of_Duty:_Black_Ops#Campaign