RC rock crawling is the opposite side of the coin for RC. It's not about speed, but about power and maneuverability, navigating obstacles and extremely uneven, rocky surfaces that many other RCs can't handle. While some RC monster trucks can handle rough terrain, a specialized class of RC Rock Crawlers are designed to perform in extreme, sometimes vertical terrain.
Rock crawling is not new. People years ago were heavily modifying monster trucks to handle the rocks, but a couple of years ago, RC manufacturers starting releasing purpose based crawlers and now trail, and scale trucks that not only look real, but perform great on the rocks.
For rock crawling you want more controlled power (torque) rather than high speed and high RPMs. Rock crawlers can use stock electric motors and lower gear setups that deliver steady power at low speeds to help get up and over those rocks.
Unlike other types of RC driving, with RC rock crawlers you'll also want locked differentials. Not all wheels are in contact with the ground or rocks at all times. If not locked, the differentials may send power to the wheels that are off the ground rather than the ones that need the power to get over the rock.
2.2 and 1.9 inch off-road tires and rims are common on rock crawlers. A narrower rim puts more tread on the sidewall and gives rock crawlers better traction all-around because the tires aren't running on a nice, smooth surface like you find on RC tracks -- on-road or off-road. Rock crawler tires typically have deep, chunky treads and are fairly soft. Although you can glue tires and rims, some rock crawlers like to use bead-lock rims. One of the advantages is that the tire doesn't separate from the rim easily when subjected to the stress of rock crawling.