Leatherworkers, Blacksmiths and Tailors can produce many items that are related to each other. They are identified by having a common prefix (eg. "Turtle Scale ..."), and they are usually all produced around the same level of skill.
They are not true sets, and do not confer any bonus for wearing many of the items. This wiki calls these "collections".
Items associated by adjective only but deriving no group benefit are not true sets but are collections, sometimes called vanity sets. These provide no bonuses when worn together other than the bonuses of each individual item, and are often worn piecemeal.
Collections usually lack the stringent definition of formal sets.
They usually have a themed look which provides a significant role play value.
These collections have many advantages over random gear:
Most of them are BoE and can easily be bought from the Auction House or a low to high skill crafter
All parts of the collection generally have similar level requirements
The materials needed to form the collection are generally farmable at the same level at which you can wear the collection
Usually, every item in the collection has bonuses to the same stats, allowing you to easily find items for each armor slot that target the stat bonuses you require
Since there is no set bonus, if you pick up a new item that is better than any single item in the collection, you can just start wearing it, and throw away the old item
Armor that drops in Sunwell Plateau can be traded in with Sunmotes to a Shattered Sun Offensive quartermaster for counterpart armor sets. Both the sets are untitled, unclassified, of epic quality, and are a notch above Tier 6 armor.