A unit of troops can often, include a single champion. He can be bought as an upgrade to a normal model in the unit as described in the relevant Warhammer Armies books. Champions are extremely skilled or tough, and comrades look up to them and take pride in their prowess. Champions of units have various names depending on the army they fight for - Orc champions are called Bosses, while Empire champions are called Sergeants or Marksmen, for example. No matter their name, champions always follow the same rules.
The champion is always armed and equipped in the same way as the rest of his unit, unless otherwise noted (even if the model is sometimes different in order to stand out). The champion is normally placed in the front rank along with characters and the unit's standard bearer and musician.
Unlike standard bearers and musicians, enemy models can direct attacks against champions and kill them - if a champion is removed as a casualty he is not replaced by another model. If the unit suffers enough wounds to kill every model in it, the champion is removed together with his comrades, being after all a rank-and-file model.
Although they normally have slightly better characteristics than normal troopers, champions are not characters but members of their regiment and always fight as part of it, moving, attacking, fleeing and pursuing alongside the unit as a whole. If the unit has any special rules that apply to it, such as being Stubborn or subject to Frenzy, then these also apply to the champion. Unlike characters, a champion can never leave his unit or join another one.
Even though they are rank & file models rather than characters, champions are subject to the following rules that govern characters (see the Characters
Champions can move within their unit in exactly the same way as characters (but may never leave their unit).
Champions can accept and issue challenges (and, of course, refuse them!).
Champions are treated as characters when their unit is hit by ranged attacks (eg, they benefit from the "Look Out, Sir!" rule, etc)