RTR - Ready-to-Run
RTR means the truck comes assembled with the motor, speed controller (ESC), servo and radio already installed and bound. You are not building a kit. But "ready to run" still needs a LiPo battery and charger (not included) and AA batteries for the transmitter. Nearly all ARRMA sold on Amazon is RTR.
BLX - the ARRMA brushless system
BLX is ARRMA's brushless power system - the brushless motor and matching ESC. Brushless means more power, more efficiency and less maintenance than old brushed motors, plus it is waterproof-rated for wet grass and puddles. When you see "BLX" you are looking at the fast, modern power setup. The number before it (like 2050Kv) is the motor's speed rating.
4X4 vs 4S - not the same thing
This is the confusion that trips up every new buyer:
4X4 means four-wheel drive - all four wheels get power. It describes the drivetrain. Most ARRMA models are 4WD.
3S / 4S / 6S / 8S describes the battery power class - how many LiPo cells it runs (see the battery guide).
So a listing like "ARRMA Granite 4X4 3S" means a four-wheel-drive truck built for 3S power. "4X4" is the drivetrain; "3S" is how hard it hits. They answer two different questions.
Scale numbers - 1/5, 1/7, 1/8, 1/10, 1/18
Scale is the model's size relative to a real vehicle. Bigger denominator = smaller truck. ARRMA runs from tiny 1/18 GROM models up to enormous 1/5 8S monsters. See the scales guide for what each size is like to live with.
Other terms you will see
EXB - "Extreme Bash", a beefed-up build with upgraded parts for hard hitting.
V3 / V4 - the generation of that platform; higher is newer with running updates.
Kv - motor speed rating; higher Kv spins faster (all else equal).
ESC - Electronic Speed Controller, the brain between battery and motor.
LVC - Low-Voltage Cutoff, the ESC feature that protects your LiPo from over-draining.
That is the whole language. Once "RTR + BLX + 4X4 + 3S" reads as "assembled, brushless, four-wheel-drive, entry power", the listings make sense.
Now match it to a truck: browse all ARRMA platforms or shop by power level.