Wyrmkin are a type of dragonspawn. All mobs specifically identified as wyrmkin in WoW are female dragonspawn, with the exception of the Coldarra Wyrmkin.
In the RPG
This section contains information from the Warcraft RPG which is considered non-canon.
Wyrmkin, also known as wyrm kin, form the bulk of the dragonspawn forces. Wyrmkin are the most common and low-ranking of the spawn, but they are nonetheless fierce opponents in battle. They are tough and hardy, but not particularly intelligent. Most lack self-motivation, requiring a leader to give them specific instructions and commands. Those who show ingenuity, superior skill, or sorcererous talent are elevated to the ranks of the flametongues or scalebanes.
Wyrmkin usually carry shields and swords (usually longswords) or spears provided by their patrons. These weapons are magical, with an enhancement or other minor feature. Wyrmkin are clever fighters, well trained and organized. Wyrmkin are trained to setup ambushes, use hit-and-run tactics, and generally do everything they can to gain the upper hand in a battle. They employ advanced battle strategies and capable of performing advanced maneuvers utilizing terrain, ambush, and flanking, as well as distinctive formations for different foes. Most wyrmkin fight in large units, they known for flanking their enemies or taking specific formations, but they rarely think of strategies on their own. Their one critical flaw is a lack of initiative and creative, independent thinking. Despite this shortcoming, they are a feared battle unit, capable of wreaking much havoc. Fortunately, most of these groups are lead by higher-ranking dragonspawn, to whom the wyrmkin looks for orders. Wyrmkin are almost always fighters, although some sorcerers are found within their ranks, but very rarely — those who show such talent are promoted to the flametongue legions.
While most wyrmkin are content — and often eager — to serve their dragon patrons, some strike out on more adventurous paths. Sometimes, a dragonspawn rebels against her oppressive culture, while other times a wyrmkin's patron or dragonflight offers its blessings on her quest. Some adventuring wyrmkin are on lengthy missions for the dragonflight, exploring new places and discovering new magic.
They speak Common and Draconic.