A cadre is a Tau military unit. Cadres are roughly the size of a company of Imperial Guard, but are closer functionally to regiments, as they are the Tau military's largest permanent fighting formation.
Cadres are composed of teams of Fire Warriors, Battlesuits, Pathfinders, and other infantry, and also have support in the form of tanks, gunships, and other vehicles. Unlike Imperial Guard companies and regiments, cadres are expected to be capable of serving almost any function, so they are fully combined, unlike the Imperial Guard's practice of organizing tanks, infantry, and artillery in separate formations and only integrating them at the battlefield.
When a single cadre isn't enough to get the job done, or where the job in question requires too many dissimilar elements to be reasonably organized into a cadre, several cadres may be combined into a contingent. Such a formation is not permanent, although when several cadres are seen to work well together during a single campaign, they tend to stay together for the campaign's duration.
Contingents may be temporarily formed into communes for even larger operations. They tend to be made of cadres from different septs.
An entire world's complement of a single caste is organized into a command, and the four commands at a single world (one for each caste) are organized into a coalition.
All of these formations are commanded by "Commanders" (Shas'O), but cadres tend to be run by younger, newer Commanders, higher command levels are reserved for more senior and experienced Commanders. Ethereals usually stay at the contingent level and above, though they may attach themselves to cadres if they feel a situation demands their personal attention.
- Hunter Cadre. The "standard" cadre is the Hunter Cadre, the combined-arms force we all know and love. Two to six teams of Fire Warriors form the core of the army, but they have pretty much their pick of whatever else they need. All other cadre types are specializations of the Hunter Cadre.
- Air Superiority Wing. Mantas and Tiger Sharks are all well and good, but sometimes the Tau just need a bunch of Razorsharks to sweep the battlefield.
- Armored Interdiction Cadre. Way back when (4th and 5th editions), the Tau could field a formation of tanks that generated additional markerlight hits each turn. No reason they can't be house-ruled into new Apocalypse games, and as an Armored Spearhead, they could be even better!
- Counterstrike Cadre. No, not the computer game. This is a formation of infantry mechanized with Devilfishes to give them mobility, similar in purpose to Imperial Guard Armored Fist squads.
- Dispersed Retaliation Cadre. The only example we have consists of sniper drones and Piranhas. Probably meant to harass the enemy without getting caught.
- Mobilized Hunter Cadre. Mechanized infantry with skimmer (e.g. Piranha) support.
- Optimized Stealth Group. All stealthsuits all the time. They sneak in and kill stuff, cover and surveillance be damned.
- Pathfinder Insertion Group. Pathfinders mechanized with Devilfish.
- Rapid Deployment Force. This was (4th-5th editions) a formation of Devilfish specializing in rapid re-deployment. They are similar to the new Counterstrike Cadre, but not the same -- the Devilfish could pick up their squads and drop them off anywhere on the field once per game.
- Rapid Insertion Force. These consist of a stealthsuit team, a bunch of Crisis battlesuit teams, and an optional Riptide. The Stealth team marks targets and calls down the others.
A standard tabletop army of Tau represents a generalist, combined-arms Hunter Cadre. Tau from different septs might paint a stripe of a particular color on their vehicles or helmets, but the color of their armor and composition of their force is determined by the situation on the ground, not by tradition.
That said, some of the variant cadres mentioned above have been represented in the rules by Apocalypse formations. They are composed of a limited subset of the units available to a Tau army, but gain certain special rules in compensation.
- Tau Cadre Creation Tables, a set of tables for randomly (or not-so-randomly) generating a Tau Cadre of your own.