Unit overview and tutorial. And differences between vanilla news - Realistic Combat (CaC ZH modification) Mod for C&C Generals: Zero Hour

Buildings are the same as in vanilla. Exceptions are: defenses, strategy center and propaganda center cost 4000, palace only 2500. Money generator buildings and hackers cost 50 percent more.

Infantry: it s bread and butter of your army. They are universal and expendable. (press X to spread them a bit to not get 10 losses for 1 unlucky explosion :D). They are stealthed when remain stationary. It is useful for ambushes or scouting. Everyone can destealth them at radius 300. Snipers and some other spotter units (listening outpost, jarmen kell,...) can destealth them at a greater range.
Standart infantry is useful for drawing enemy attention, firepower and killing other infantry and light vehicles, they are also quite useful against the buildings (like in vanilla? :D) (well, it isnt realistical that some rangers would be shooting to a building 3 minutes, imagine that they would hit some ammunition crate or throw a C4 here :D)
Rocket Infantry fires Armor Piercing rockets with short effective range. They can knock out most tanks in 1 hit. Very useful for ambushes and overrunning enemy. They can also hit aircraft flying DIRECTLY overhead. USA deal most damage, china in the middle, RPG troopers the least. (Toxin tractor and dragon tank have increased resistance to AP rockets, also upgraded USA tanks with +defense strategy plan can take more hits).

Tanks: they are main battle units. They can or cant be cost-effective, it depends on micro, faction, support. Anyway mix them with infantry and spread them, or form a line, use terrain advantage and watch out for friendly fire!Vehicles in general have slightly more vision radius than infantry. 
AA machine gun vehicles are also very effective infantry killers, their heavy MG usually outranges infantry, however, they need to reload/cool down they guns so even the best one can be swarmed by masses. 
Technicals and some other fast vehicles are now slightly slower. They are still useful for transport (terrorist from start) or with their speed; drawing enemy salvo while your main battle group advance.
Rocket Buggy/Inferno tank/SCUD launcher/Tomahawk have 50 % more range than tanks, also they can fire over hills (RB needs flat terrain to shoot but is fast). 2+ Infernos can randomly create powerful firestorm. SCUD launcher/ Tomahawk: they looks good, they range, tomahawk guided missiles, damage, but you should plan ahead, they slow reload speed can mean that they wont even pay itself off. Or that they are too few to cause serious damage.
Bomb Truck: I once defeated 3 overlords with them when i ran out of tanks. They have sometimes some cost-effective uses.

Aircraft and helicopters see slightly further than vehicles (even more than infantry) and is in general faster than in vanilla. Mig is now attack ground only. Raptor is more useful on larger maps, like interceptor aircraft, however it does have guided missiles, (which can sometimes guide to another nearby target :D:D:D). Aircraft often needs several missiles to be put down. (Imagine that it does have flares lol :D).

Nuke is the most powerful superweapon now, but also costly, and its the best saturation area weapon, it can kill infantry in that HUGE area even after 2-3 minutes.

USA defenses are pretty standart ones, and powerful. GLA defenses needs some placement, Stinger Site downranges  tanks so it could be placed behind some hill or cover, or cloaked. China has defenses only for specific purposes. Gattling Gun armor is weak (like patriot missile one). Bunker has the most hp from structures, it can even survive 2 nukes, but only infantry can shoot from inside so its short range and stealth make it suitable for ambushing or just delaying the enemy.

Factions in general:
USA
+most expensive units, stronger late game due to strategy center battle plans (then infantry can outrange the enemy infantry, also tanks do, ...)
+ stronger base defenses, with adequate range, early game turtling is possible

+ chinooks of course :D

+ have most aircraft- most expensive doesnt always mean the best, they can be countered or swarmed

CHINA
+ effective early game red guard spam

+ group bonuses, but: 

- (spread out units are less likely to be hit)

- base defense structures are not awesome

GLA
+ countless nasty terrorist tactics :D, specific units, etc. standart stuff...

+ tunnels

- lacks aircraft

- scorpion has slightly lower range than other tanks