Armors are distinguished by class (Padded, Piecemeal, Heavy, etc), and each armor affords different amounts of armor points (AP). The most protective armors also confer a soak. If you meet the Mighty requirement for the armor, you benefit from its protections.
In the Character Creator, you are able to choose a type of armor when you purchase armor from a class. For example, if I purchase a Heavy armor, the Creator will ask me to select a type of armor (Brigandine or Splint Mail, for example). Mechanically, there is no difference between types of armor within a class—the choice is purely cosmetic. You can ignore armor types if you're not using the Creator.
Armor classes have a Mighty requirement, which means your Mighty needs to be equal to or greater than the Mighty requirement in order to benefit from wearing the armor. If your Mighty is too low, then the armor affords no AP or soak.
Magic Point (MP) Penalty
Wearing heavy armors is disruptive to spellcasting. You can cast spells and wear armor, but if you do so, these armors impart a magic point (MP) penalty that increases the MP cost of any spell or maleficence you cast. The MP penalty reflects how exhausting it is to think magically while sweating the weight of all that gear. For example, if your armor's MP penalty is 3, then any spell you cast costs 3 more MP. This penalty stacks with shields.