The army list for each army includes options and upgrades for characters and units of troops. All of these cost extra points to reflect the increased worth of the character or troops.
For example, characters can carry extra weapons or extra armour. They might ride some kind of mount, a boar in the case of Orcs, or they might ride in a chariot or on a monster.
In the case of characters that are also Wizards (Shamans in the Orc army) they can also increase their magic level at additional cost. Characters can also carry magic items to a predetermined total points value, and the more powerful the character the greater the total points value of magic items he can carry.
Units of troops can also be given extra weapons or armour. In the case of units all the models are given the same options automatically, so the total cost will vary depending on the size of the unit. In addition some troops have unique options depending on their type. For example, one unit of Orc Boyz can be chosen to represent 'Big 'Uns' - the best warriors in the tribe with improved characteristic values.
In most cases a unit of troops can also have individuals upgraded to represent a Champion (called Bosses in an Orc army), Standard Bearer or Musician. Champions have superior characteristics and cost extra points as you would expect.