Dragon Slayers are a unique type of people who utilize a form of Lost Magic, Dragon Slayer Magic. Each Dragon Slayer has their own form of Magic, just like each Dragon is a master of their own element. In addition, they also have the ability to consume the element which they wield, just as long as it is not created from their own Magic. Doing so rejuvenates them and enhances their abilities. Dragon Slayers are known to possess keener senses than those of normal humans such as in smell, sight and hearing, as well as distinctive characteristics and features such as slitted pupils and noticeably sharper teeth.
Furthermore, if a Dragon Slayer manages to successfully kill a Dragon, they become stronger after bathing in their blood. However, Dragon Slayers can transform into Dragons themselves through excessive use of their Magic; the only one who is known to have killed Dragon is Acnologia, who killed nearly every single Dragon in existence, but as a drawback was transformed into a Dragon who was also "not quite a Dragon". The only way to prevent this "Dragonification", as it were, is for a Dragon to enter the body of a Dragon Slayer via a secret art and sleep there, creating antibodies as they do so. After enough time has passed from the point of their creation, the Dragon Slayer is almost guaranteed to never turn into a Dragon.
There are three generations of Dragon Slayers: the First Generation, who learned their Magic from actual Dragons; the Second Generation; who have had Dragon Lacrima implanted into their bodies; and the Third Generation, who are a combination of the previous two generations.
It seems that, for some reason, all Dragon Slayers of an advanced "level" suffer from severe motion sickness. At lower "levels", this isn't much of a problem, as shown with Gajeel prior to the year X791, having rode on both the Super Mage Giant Phantom MK II and the ship to Tenrou Island with ease. However, after three months of intense training, he, too, became vulnerable to motion sickness, having trouble running on a chain of vehicles during the Chariot event. Unless the vehicle is specially designed to accommodate Dragon Slayers, or if they believe the transportation is more of an ally than a vehicle, they will almost definitely experience motion sickness.
The First Generation Dragon Slayers were the original Dragon Slayers, who learnt their Dragon Slayer Magic from an actual Dragon. In most cases, the particular Dragon Slayer’s teacher was also their foster parent.
Those from the second generation of Dragon Slayers have had Dragon Lacrima implanted into their bodies, granting them the ability to use Dragon Slayer Magic. Because they were not taught their abilities by a Dragon, they are considered artificial Dragon Slayers.
Dragon Slayers heralding from this generation have both been taught their Magic by actual Dragons and have had their bodies implanted with Dragon Lacrima.