Heart of the Swarm introduces Primal Zerg. What are they?
Source, SC lore (and the wikis, of course!)
BIG BIG BIG SPOILER (not really, you can find it in the wiki):
The primal zergs are the zergs that did not get corrupted by Amon (or whatever how we write the name of Dark Voice).
They are the original zergs that were made by the Xel Naga. The zergs we know that can be part of the Swarm were corrupted to be made into weapons. All the zergs in the swarm share a psyonic link with each others and that makes them dependent on their leader. A leaderless brood isn't more dangerous than wild animals (as can be seen in Flashpoint or the second book of the dark templar saga).
The primal zergs don't have that psychic link. Packs are made like wolf packs where there is an Alpha male (Brack in the Zeras mission is one of the alpha). From what I can see in the mission, most current Zerg units seems to have evolved from the primal zergs in some way. The primal zergs are also independent and only respect power. Also, each primal zerg have the ability to indivually evolve by absorbing essence while the swarm has to go through more steps to evolve.
tldr; They are the original zergs. Most zerg units have evolved from their primal zerg counsins.