An army composition list is broken down into a number of categories, each listing the units that belong to it:
Characters: The characters that lead your army, powerful individuals possessed of fearsome martial or magical might.
Core: The heart of your army, the iconic troops who make up the bulk of every warband and warhost.
Special: Skilled and veteran troops, capable of anchoring a battle line of lesser warriors, or performing great deeds in their own right.
Rare: The most unusual models in your army, mighty monsters, weird war machines and elite soldiers of unsurpassed skill.
In addition, many army composition lists will include one or both of the following categories:
Mercenaries: Some armies can include specific units drawn from other army lists as mercenaries.
Allies: Most armies can include an allied contingent drawn from another army list.