Armor is currently available in 3 different weights:
Each weight has its own skill line and profits from various passive abilities, too. light armor utilizes magicka, whereas stamina setups benefit the most from medium armor, and tanks usually want to use the heavy armor skill line.
Even your Champion Points can influence the type of armor you are wearing.
One of few exceptions are the so-called Monster Sets. These are available as head and shoulder slot only. The helmet drops from veteran dungeons at the endboss and the shoulder drops from the respective chest at the Undaunted Enclave (consumes one Undaunted key).
If you want to know more about it, check out this page about Monster Sets. It will show you the bonuses, the location where it drops and from which chest.
Note: The quality of armor is not as important as the quality of weapons. The increase in stats that you gain from example purple to gold quality is in most cases very tiny.
The picture below shows you an example of a set that is called Poisonous Serpent. the picture shows a gold quality medium armor chest with a divine trait and a max stamina enchantment which is very common among melee fighters. You can find all available traits for armor here: "Pick your Traits".