Common Historical European Armor Types and Playing them in UO
First of all let me say this: There's no actual requirement that armor in UO be realistic or that, if it is realistic, it matches European models, and both are good things. I like the diversity of styles we have in this game.
For all we know a given suit of armor your character has could rely primarily on magic for protection. Or it might not actually be armor at all, just a uniform, costume, or regular clothes, and a character's protection might come mostly from dodging, ducking, blocking with shield or weapon, and whatnot. Or your armor might have some kind of semi-magical technology that makes it work.
The possibilities are endless, and it's not up to me to tell you to or not to do certain things in how you play your character.
However....Some of you though might be like me and just like the idea of your character's armor being realistic. Maybe it adds to the common-ness of your character (like Sam Winchester said, “I’m just a guy”), or maybe you like the idea that your character could be transported to the RL Middle Ages, or a low magic world like George Martin's Westeros, and still get by as a warrior of some kind.
And for those folks, here is some basic information on historical European armor types and some very basic suggestions on how to portray them in UO. I'm focusing on European armor types simply because I'm more-familiar with them than with other cultures’ armor. (So the very common Asian armor type called “lamellar,” though a great armor design and worn throughout Asia during the Middle Ages, isn't covered here beyond mentioning it a couple of times.) I'm focusing on the Medieval period because most Western fantasy is based on that period (sort of anyway...). I'm also painting in very broad strokes here and there're a lot of nuances that I won't be catching. I also won't be talking about helmets very much. Which is a shame but when I tried to include helmets the article got too long. Also a lot of us just like wearing various kinds of hats anyway and in UO there’s rarely any functional difference. And, since it wasn't uncommon to wear a helmet under a hat (sometimes this was called a “secret”), or to style a helmet to look exactly like a hat, there's a pretty extreme amount of variety when it comes to helmets.
A lot of information about historical armor is in flux because a lot of work is still being done. For example, for a long time it was assumed that artistic depictions of armor from the Middle Ages were mostly fantastical in nature. Then, just a few short decades ago, someone got around to designing some real life armors based on the artwork and, a lot of the time, the artwork was proved accurate. And now you can have real armors based on artwork, or knights’ funeral effigies. And sometimes you can actually find a historical suit of armor that artwork was based on.
Finally I also note that I've never studied this stuff professionally and am relying on a very uneasy combination of academic and non-academic sources. If you get curious and start reading (I got some suggestions below), stuff you find may differ.