Tau comprise of 5 castes; Air, Water, Fire, Earth and Etheral.
Water Caste are the diplomats and merchants of the Empire, these are the ones who talk to other races regarding joining the tau empire and negotiating trade deals and the like. Very often the first face any race will see of the Tau Empire (and of the Tau race itself). If I remember right, they are sleek and smooth in appearance and have a natural charisma befitting of such position.
Air Caste are the pilots of the many aircraft of the Tau Empire. Be it Zero-G or atmospheric, these guys fly it. Their most notable trait is vestigial remains of their wings they once had back in their primitive state as they used to fly by their own ability. Very thin and tall, very brittle looking.
Earth Caste can be best described as Tau Dwarves. Shorter but sterner built than any other caste, these are the builders, scientists and engineers of Tau. Any gun, armour or battlesuit you see the tau use was created by an earth caste member. These are the ones you can thank for all them sweet battlesuits.
Fire Caste are the face of the Tau empire on the board as these are the warriors of the empire. From the humble Shas'la to the venerable Shas'O, you know what they look like thanks to the models and are the most warlike of all the castes being naturally more aggressive than the other castes and having the build for it.
Etheral Caste just showed up. Shrouded in mystery these figures just arrived one day, walked into cities, said "for the greater good" and everyone went "yup, sounds good" including even the war like Fire Caste. These are the ruling caste of the Empire and wield a potent level of control over other tau, whether they are Psykers of any kind is up for debate however personal conjecture is I lean in that they are Psykers but in the same scale that tau have warp presence which is near nothing.
Lore wise, born to a caste means you stick to it and do what you can for the empire. Always for the "greater good". Naturally we have more information on the fire caste than any other.
In terms of how they treat their own kind, there is a mixture of grim dark coupled with noble bright elements that I think if fleshed out better would create a very compelling race with an appealing outside but their deeper lore being just as sinister as the imperium. Generally, factions brought into the Empire are treated well however there are fringe cases where the darker elements show through, including on their own native race of Tau.
In regards to Gue'Vesa (humans who have defected to the greater good), this can be a bit of a mouthful to figure out fully. For what I am aware these individuals still get to worship the Emperor however as long as they keep pushing towards the greater good. The Tau often regard humans with a wide degree of emotions, fear and respect being chief among them. They see their ability for war as astonishing along with their weapons being primitive but just as lethal if not more than their own. For Gue'Vesa, they can take up roles within any caste of Tau society I believe with the obvious noting of them being in the military where they aren't forced to use old gear and are actively armed with Tau equipment.
In terms of gameplay, there are various routes to take. So lets put down some special mentions.
Coldstar Commanders. The mucho grande final word in what commander you want to take when going stock codex. Mobility unrivalled by anything that isn't a Flyer force org slot and the ballistic skill to back it up. Excellent to keep as pocket units ether for later in the game or if want to, the now fading from popularity fire and forget missiles that are quad fusion commanders. Never a bad choice, NEVER, and you often want to take triple battalion so you can indeed have 3 of these fly boys doing their thing.
Strike Teams. Love how people call these guys squishy when they have a 4+ armour save. Slap them in cover with their pulse rifles and unless some AP comes their way they have a 3+ save which is nothing to scoff at. They aren't brick walls but you may be surprised at how well they last when it gets down to it. Often taken in 5 mans with the sergeant having a markerlight for cheap and cheerful troop choices that just overall get work done.
Riptides. Riptides...what do I need to say? Take 3, watch them get work done and always keep shields up at all time when you can. Fast, shooty and tough as nails.
Pathfinders. Don't take them for markerlights, take them as special weapon teams as the rail rifles and ion rifles they can take often do a lot of work for the cost and considering these boys aren't too expensive, sometimes popping the surprise 6 rail rifle shots will get a reaction.
Cadre Fireblades. Take them, you don't have a choice but don't worry these guys are worth it if just for their Ballistic Skill paired with a markerlight and when enemies get close to strike teams with him around, suddenly the number of shots skyrockets!
For colour scheme, pick whichever colours you like. There are some set sept colours but Tau are notable in not sticking to it as much as marines or guardsmen with the only designator for sept being colour used for their squad markings (as far as I read/was aware). Talking of septs:
Vior'La if you want to get aggressive.
Bork'An is a bit more specialised but has applications, steer clear until more aware of how Tau works to learn why this sept can be good.
Dal'Yth. Want cover? Cool, here. Done. Simple and they also have a very powerful relic for commanders.
Sa'Cea. Leadership boost (...yea...such boost...such merit...). The main draw is the fact that every unit now gets 1 re-roll to hit which can be very useful for priority shots (like markerlights from the sergeants)
Far'Sight for those who love their lore but sadly terrible trait unless you go for specific loadouts that currently don'y do well. Pick them if you love them but remember, no etherals.
T'Au. When you charging is getting you down, 5+ on overwatch. The default go to for that reason and the fact they have the 2 best HQs in tau (darkstrider and longstrike).
Do you but there's my thoughts for you.