Gue'vesa
The fucking Space Communists are a bit odd in that they aren't alone in their cause; unlike every other xenophobic shit in the 41st millenium, the Tau have one area where they aren't fucking retarded: the fact that they allow non-Tau to serve the empire. Other races abound in the Tau empire, including Kroot and Vespids, but the best-known (and frankly most compelling) of the set are the humans in Tau territory, who wound up joining up with the Tau cause. Humans working for the Tau empire are called "Gue'vesa," which quite literally means "Human Helpers" in the Tau language (the Imperium's grunts however joke it means "Human Cannon Fodder" ironic considering that they themselves are cannon-fodder).
There are two divisions of the Gue'vesa encountered; those maintaining self-contained armies on behalf of the Tau (those using the Imperial Guard codex), and those using human auxiliaries to Tau forces (via chapter approved rules that are now of questionable tournament legality). It is also possible to represent them by using the Tau Codex and simply replacing the Tau models with human variants, and this is legal, but it will result in bricks being shat by other Tau players.
The Tau term for Gue'vesa soldiers is "Gue'vesa'la," and the Tau term for their Sergeants is "Gue'vesa'ui." Similarly, high-ranking ones are "Gue'vesa'vre" and Commanders are "Gue'vesa'el." High-level Gue'vesa commanders (ones comparable to a planetary governor) are called "Gue'vesa'o."
Inquisitors call them "heretics and traitors." Angry Marines call them "Weeaboo faggots."
How This Happened
The Tau had finally gone and pissed the Imperium of Man off but good, as they had established a series of trade agreements with Imperial Worlds on the frontier, near the Damocles Gulf - and had begun trading goods and technology with the humans of the frontier. Naturally, since the Imperium immediately refers to any contact with Xenos that doesn't involve killing them with fire as Heresy, the Administratum flew into a rage that would make an Angry Marine titter with schoolgirlish glee were they capable of such. After fucking around for a while they eventually sent a hugeass fleet into the sector about a century later and began blowing the fuck out of everything and pushing deep into Tau Empire territory.
The Imperial rapetrain ground to an abrupt halt when they came across the Dal'yth sept, which basically decided to do a bang-up impression of the Imperial Guard and hold the fucking line. Simultaneously amazed and horrified by the prospect any race, let alone a bunch of blue-skinned unguligrade alien space communists, could have the mighty balls to stand up to the Imperium in a stand-up drag-down fight, the Imperium was momentarily given pause - before it resumed doing its usual thing and blowing the fuck out of its enemies.
Sadly for the Imperium of Man, the Tau did not relent, and showed a balls-out stubbornness that would make any Commissar shed manly tears of pride and RAGE. Even still, with the Imperials having a mighty hard time gaining ground on the Tau - the space communists had superior firepower and were fighting defensively, which was a fight that was not very much to the Imperials' advantage, what originally would have been a fairly short affair wound up embroiling the entire sector in a lengthy, drawn-out conflict. The Imperium was winning, slowly but surely, but at the rate they were progressing, it would be an eternity and a day before they could claim to have won the war - and likely trillions more lives. Moreover, though the Imperium had a gigantic manpower advantage (one that got bigger as the war dragged on), the war basically stalemated (Imperial Commanders blame Fish of Fury for this), with gigantic losses on both sides wearing down both the Imperials and Tau.
After it became painfully apparent that both sides were getting absolutely nowhere this way, the Imperium eventually relented and entered into meetings with the Tau Empire for cease-fire talks. Whilst neither side trusted the other one bit (this is the universe of grimdark after all), the meetings were basically successful, in no small part due to a Tyranid Hive Fleet (specifically Hive Fleet Kraken) moving in to rape a huge shitload of strategically-vital territory - followed immediately by a second hive fleet (Behemoth). This forced the Imperium to stop trolling the Tau diplomats and agree to the fucking cease-fire in order to be able to go fuck that shit up before things got even more grimdark. They still fight, of course, especially when there's a conflict of planetary interest, but it's mostly individual battles rather than protracted war.
This sudden rapid deployment left a shitload of Imperium personnel stranded, especially those that had been captured by the Tau, and a lot of humans remained in formerly Imperial territory that was now well within Tau space. Most of these humans were offered an agreement by the Tau to join the Tau Empire, and a great many did so, as the choice was between the POW camp or becoming Tau Empire Citizens. Like a choice between fine wine and a toilet full of spoiled meat, this was not exactly a difficult fucking choice.
The Gue'vesa
The Gue'vesa are basically Imperial Guardsmen working for Tau. There are two versions of them - auxiliary troopers working for the Tau Empire (which can be fielded in a Tau army), or actual Imperial Guard regiments (made using the Guard's codex) that are just modified to look a bit more Tau-flavored and are used to represent Gue'vesa troops serving as colonial Planetary Defense Forces (for example). The Tau have generally kept a pretty good relationship with the humans in their territory (the Tau scenario in Dark Crusade has them sterilizing rebellious humans, but this is non-canon to all true fatguys, though fun to make fun of), and the biggest fears that most humans in Tau space have is the Imperium getting pissed enough to come back and try to finish the fucking job.
Those humans who have joined the Tau empire have been provided with the technology required to prevail on the Tau frontier. They have access to their own production capabilities, allowing them to construct equipment ranging from farming and mining tools to laser weapons. Gue'vesa actively working for the Tau are also capable of using Pulse weapons, whilst Gue'vesa assigned to planetary defense duty generally lack these - but have access to your usual stock of fuck-your-shit up ordnance.
Gue'vesa auxiliaries working for the Tau are an unusual unit; mostly armed with Lasguns, they provide Tau with a combination of numbers and slightly better melee capability. One nice thing is that unlike the Kroot, the Gue'vesa aren't morons, and thus can equip specialized Tau-manufactured equipment, such as Markerlights, EMP Grenades, and pulse weapons. The fact that you get pretty good numbers per squad and the fact that they're kinda inexpensive makes them a pretty nice thing to have for a Tau army low on its numbers. Their one drawback is that they automatically are favored enemy for Imperial troops, who will hit them automatically in close-combat on a 3 or better.
In spite of this, Gue'vesa make a damned fine Tarpit unit. They are shooty enough to actually cause some damage to squads that aren't ready for them, their Gue'vesa'la can drop a Markerlight, and they cost notably less than even Kroot Carnivores. They also have an armor save (however shitty). They cannot infiltrate or use transports (which sucks), but their sheer cost and mass makes them a damned fine meat-shield unit for those who aren't terribly fond of using blobs of Kroot. Gue'vesa that do well enough are frequently given better equipment and promoted, with veteran Gue'vesa supposedly being eventually allowed to join Fire Warrior teams.
Gue'vesa armies made using the Guard codex are Imperial Guard units in every way but name and appearance.
This is basically also true of those playing Gue'vesa using the Tau codex.
Gallery
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The Imperial Infantryman's Uplifting Primer on the subject.
