The rebels of Warpath, written as Rebs in the fluff, are a coalition of numerous alien races that oppose the Corporation. They're said to be a secret resistance, although they've been fighting the Corporation for centuries so it's hard to say how secret they really are. Later editions have retconned this. While they have secret cells within GCPS space, reb also control their own homeworlds and while not recognized by the GCPS as a state they are essentially a de facto confederation of aliens and humans who got pissed by the corporation evil expansion. Unluckily, due to most of the fluff being writen in corporation POV it is hard to know if any of the other great races recognize them as a power in the galaxy, well at least the organized ones like Forge Fathers, Asterians, Nameless and Z'Zor. I doubt being recognized by the Veermyn or the Plague would ammount for anything in the warpath universe(if that is even possible)
Ok so, living in the GCPS is not really as bad as living in the Imperium, by a mile, even aliens can archieve a certain degree of respect and have representation in the council. Yet, some worlds are well, not pleasant, and also the corporation is keen on exploiting aliens when profits are endangered. Thus the corporation has been part of several double crossings, environmental destruction and even genocide and the worst of the worst, TAXATION on humans(the bastards).
Expectedly this pissed of several races, and humans, especially the latter, as the formal date for the begining of the rebellion is related to a TAX revolt(yup mantic went there), soon the human rebels found alliances with alien races well spread accross the GCPS, from nearly wipped out Yndiij( whom the Rebels actually saved from beign whipped out) to "make the teraton empire great again" teratons(who disagree with how the Teraton empire has been reduced to a puppet state within corporation space) and Judwan (but they dont fight, so who cares).
Overall the rebs were able to liberate a piece of space within the frontier and establish a lose federation, they would probably have been crushed byt the GCPS were it not for them beign busy fighting other dangers,(of the we spread like the plague nature).
Their forces dont form a standard army as well as other factions, and most of the battles in which they are engaged are guerrilla warfare. The loose structure of the army means that reb forces tend to have different types of aliens within the same squad, hence making them a modellers dream, if your dream is bringing variety to the tables.
If 40k comparisons were allowed, they are similar to the tau empire, beign both a confederation of mostly peace oriented people, Judwn even look like air caste tay. Only scratch empire because rebs want no part in imperialism(probably then again, having only a GCPS POV makes it hard to understand the rebs entirely)
Their armies include both overt armies fighting on distant planets, as well as saboteurs hidden within Corporation ranks. On the tabletop this is somewhat represented by the difference between Deadzone and Warpath forces, although you can field any of their infantry in both formats.
Their name "Rebs" comes from a pejorative term used by Corporation forces to refer to them that they later adopted as their own. In the same vein, they call the Corporation the "Evil Empire." Yeah, wonder where they got that from...
The Unincorporated Rebs team of Dreadball was introduced in the Season 4 book. They're kind of an odd team in that they can customize their players to some degree by using different member races in different player positions. The kit comes with two each of Gaelians, Ralarats, Rin, and Soraks that can be run in different roles.