The Vok were mysterious energy-based alien creatures, enlightened descendents of The Swarm, who developed prehistoric Earth towards their own designs to atone for their shameful past. The Vok were dedicated to "the Project", which consisted of enigmatic, sometimes contradictory plans for the planet. As a result of these experiments, there were a number of anomalies on the Earth, including several floating landmasses, a mysterious stone formation, and a hollowed-out moon.
Their experiments were irreperably wrecked as a result of the Beast Wars. The Vok decided to cut their losses by revealing the nature of the second moon: An immense cannon designed to destroy the planet! Optimus Primal sacrificed himself in order to destroy the Planet Buster.
As a result of the Planet Buster's explosion, a Quantum surge was released. The surge caused a mutation in several of the Transformers (notably, Megatron, Rattrap, Cheetor and Tarantulas) turning them into Transmetals. The destruction of the Planet Buster also released a Transwarp wave that traveled through space and time, and eventually got back to Cybertron
Later in the series, the Vok made several other attempts to wipe out the Transformers, notably a second weapon which appeared on the planet (sometimes referred to as the Metal Hunter), and the abduction of the Maximals Airazor and Tigatron.
Near the end of the Beast Wars, the Vok sent an emissary, Tigerhawk (a newly created fusion of Airazor and Tigatron), to Earth to deal with Megatron's disruption of the timestream. Tigerhawk was able to destroy the Predacon base, but was eventually subdued and captured by Tarantulas. Tarantulas was able to draw the Vok out of Tigerhawk's frame and into his own, but this lead to his own destruction...
Primeval Dawn comic
...and subsequent Vok-powered rebirth.
With the power of the Vok at Tarantulas' fingertips, the alien race turned to the Ark, creating Primal Prime (from The Matrix, a Datasphere and Quickstrike's control harness) and restoring the sparks of Airazor and Tigatron to separate Transmetal bodies, hoping they could defeat Tarantulas. Later when lost the Matrix to the zombified-arachnid they merged themselves with Primal Prime in order to provide a temporary substitute to the Matrix. Primal Prime gained a snazzy new Vok-shaped breastplate out of the deal.
The ending of this story was long promised but not yet delivered when 3H lost the liscense.
In the course of writing the Beast Wars television show, the co-story editors had conflicting ideas on who the Vok were. Bob Forward liked to imagine the Vok as the evolutionary endpoint of all sentient life in the universe, shaping the development of other races in order to eventually guide them into becoming Vok themselves. Larry DiTillio wanted to link the Vok to The Swarm, who, in his version of the events of the Generation Two comic book, had completely wiped out humanity and wished to atone for their crimes. Neither origin made it into the television show before it completed.
Ultimately, the Vok origin was published as backstory for OTFCC's Primeval Dawn storyline.