Starter Cultures vs Adjunct(Secondary) Cultures
Starter cultures are there to start off the process of cheese making and their first and primary job is to form lactic acid from the lactose to inhibit other organisms(not so happy in acid environment) from eating their food.
Further on during the process these starter cultures will by enzymatic action break down the proteins and fats to give flavor and cheese texture.
Adjunct(secondary) cultures are not in most cases fast acid formers, but they have other talents ;-)
For example, Lactobacillus helveticus is super proteolytic, thus giving cheese a softer smooth build faster than most other cultures. Also it has the ability in most cases to consume galactose. This helps in some cheeses to prevent excessive browning in the oven.
Propionibacterium is a secondary culture that forms the holes in Swiss style cheeses.