For some historical reading on this subject I highly recommend
Sengoku Daimyo's Japanese armour overview. I especially recommend his page on
various dô styles; it does a good job of explaining what terms like Mogami, Okegawa, etc. all actually mean alooked it over very interesting i liked how the author described in detail how each piece was laced and the thought of all the research that went into it all.nd come with very helpful pictures. Generally these terms refer to how the armour is laced and the overall appearance.
In terms of gameplay, the order from highest to lowest defence for torso armour is:
Nanban > Yukino****a=Kozane > Nuinobe=Hishinsui > Okegawa > Mogami > Okegawa Retainer=Faction Retainer=Okegawa Hara-Ate > Tatami > Tatami Hara-Ate
Note different armours have different strength requirements. Nanban, Yukino****a, and Kozane require 10 strength, Nuinobe and Hishinsui require 9, Mogami, Retainer, and Okegawa need 7. I may not be remembering everything exactly accurate, but that's a rough estimate of the order.
For helmets, the more enclosed they are, the higher the defence and weight. Most helms come in three versions that have progressively better defence: an open faced one, a version with either a mask or a maidate (decorative horns/crests), and a general's version that usually incorporates both a mask and maidate (the general versions are usually indicated in the name).