Warbands are simply scaled-down Warhammer armies. In essence, Warbands are the same as standard, tournament-sized armies but are simply in a growing stage.
The term Warband describes a Warhammer force in the range of 100 to 500 points. Warbands are not quite large enough to be called armies but are big enough for unit-level gaming (as opposed to gaming on the individual-warrior level, as is the case with games like Warhammer: Skirmish or Mordheim). Players can imagine their Warbands as patrols, reinforcements, special contingents sent off to accomplish specific tasks (e.g., raids, sabotage, assassinations), or simply a small force in the growing stages.
Click on the image for the pdf rule set.