An RC truck works much like a real car, but instead of a driver, it receives signals from a hand-held transmitter. The transmitter's joystick or wheel controls the throttle and steering. The receiver in the truck tells a small electric motor to spin the wheels. That's really all there is to the basic concept. The differences between models come down to size, power, suspension, and intended use.
There are four main types of RC trucks beginners will see: monster trucks, stadium trucks, buggies, and crawlers. Monster trucks have big wheels and high ground clearance, making them great for grass and rough ground. Stadium trucks are similar but lower and more nimble, designed for dirt tracks and jumps. Buggies are smaller and lighter, good for hard surfaces and packed dirt. Crawlers are slow and incredibly precise; they're designed to climb over rocks and logs without rolling over.
For a complete beginner, the best choice is usually a monster truck, a stadium truck, or a small crawler. Monster trucks and stadium trucks are easy to drive fast and feel exciting. Crawlers teach you throttle control and are very forgiving because they don't go fast enough to crash hard. All three give you an excellent first experience in the hobby.
When people say "beginner-friendly," they mean a truck that forgives mistakes. It should not break when you run into a curb. It should be easy to see at a distance. It should not flip over at the slightest touch. And it should have parts available in case something does break. A beginner-friendly truck is also simple to charge and doesn't require a pile of tools to maintain.
A common marketing term is "hobby-grade," which usually means you can repair and upgrade it. Another term you'll see is "brushless motor." Brushless motors are faster, more powerful, and require less maintenance, but they are usually found on more expensive trucks and can be too fast for a beginner. "Brushed motor" trucks are cheaper and a little slower, but still plenty fast for a first truck, and they're easier to control.
Your first RC truck doesn't need to be the fastest one on the market. A top speed of around 20 to 35 mph is more than enough to have fun and learn. A truck that can do 50 mph out of the box is almost always a bad idea for a beginner, because crashes at that speed cause expensive breakage and can be dangerous.