The Saurians are reptilian humanoids who evolved from the dinosaurs. Standing on two legs and having bigger brains than their ancestors, they have the potential for more complex thought. They can also appear to alter their form to become invisible or look human.
The movie Mortal Kombat depicts that Saurians as, demonstrated by Reptile, have the ability to shapeshift into statue form (or just look like one) and their true Raptor form. They also possess the ability to blend in with their environment through the use of a highly effective camouflage that renders them almost completely invisible.
Mortal Kombat: Conquest also depicts their diet as consisting of live bugs and worms, served in a large bowl, as revealed in the episode In Kold Blood when Reptile ravenously consumes his squirming meal in a large, clay bowl during his candlelight dinner with Kreeya.
They have a matriarchal society, and any Saurian that has drifted away from the influence of the matriarch or a surrogate slowly begins to lose their ability to look human. Their saliva is comprised of powerful corrosive fluids that can easily burn right through steel or human flesh, much like acid. This saliva is so powerful that it even kills other Saurians, as demonstrated in Mortal Kombat: Conquest.
The Saurians were originally native to Earthrealm where their civilization thrived countless years ago. However, their age came to an end when a war raged amongst the Elder Gods. At the time, it was the God of Darkness Shinnok who had turned against his brethren in order to become the master of the realms and sought to possess Earthrealm. He was opposed by the Thunder God Raiden and the conflict between the two was so devastating that it ravaged Earthrealm. The Saurians managed to flee the devastation and left to another realm known as Zaterra where they attempted to rebuild their civilization and for a time it began to thrive once more.
Eventually, the Emperor of Outworld known as Shao Kahn invaded the realm of Zaterra where he swiftly conquered it. As many other realms before it, the lands of the Zaterrans began to merge into Shao Kahn's own kingdom with the Raptors driven to extinction except for a few members of their kind. Among the survivors were Reptile and Khameleon; the first of the two decided to serve Shao Kahn whilst the latter intended to get revenge for the destruction of her kind.
Reptile's Armageddon ending shows another female Saurian, possibly the race's long-dead matriarch, which Ed Boon has confirmed will play a big role in restoring the Saurian race in future games. In alternate timeline of events, a possible outcome led to Reptile turning against Shao Kahn and slaying him afterwards he forced Shang Tsung to use his Flesh Pits to restore the Raptor race. After many agonizing attempts, he succeeded in creating the broodlings that were loyal to Reptile and eventually they stormed Outworld; slaying every Tarkatan, Shokan and Centaur who objected to Reptile's rule.
Behind the Scenes
A number of Saurians also feature in an episode of Mortal Kombat: Defenders of the Realm  Although Reptile himself was not among the group, he was mentioned by name after their true nature had been revealed. This episode coined the term "Raptors" for Reptile's race which was largely considered the canonical name for the species until the term Saurian was introduced in the games.
They also featured in Mortal Kombat: Conquest including the episode "In Kold Blood" where they are referred to as "Reptiles". There, they are shown to had allied themselves with Kreeya and were shown to alternate between their saurian dinosaur forms and their human disguises. Here, it was shown that Reptile had allied himself with Kreeya against the wishes of his people in order to aid her in defeating Shao Kahn and thus stationed several of his warriors at Kreeyan hives to protect them from intruders.