Rally races can be held on pretty much any drivable surface, but the fundamental motivation behind rally strategy comes more from the unknowns of driving a new course in every race than the characteristics of any particular road surface.
To elaborate on that statement, road racing (at least as I’m using the term in this answer) is done on a closed circuit where drivers practice the course that they will qualify on and race. In rally, the drivers have not driven the course before the race other than possibly a slow run to see the course once (“recce”/reconnaissance). This is what motivates the primary strategy difference: racing line.
I’m going to assume if you are interested in auto racing you know the basic concept of a racing line. In general on a road course, there’s going to be a pretty clear “fastest” or “ideal” line to follow for a given car that will get the fastest lap time. Obviously every driver tries to follow this line for their car. Since there are other people on the course and drivers have to pass each other, they may adjust the line they follow to gain a temporary advantage for a pass or to block other drivers from passing.
In rally, passing is so rare that it does not influence the fundamental driving strategy to the same degree as road racing. In rally, what the drivers are looking for in their racing line is flexibility. For each corner, the driver will, at best, be told the direction, severity, and possibly some extra cautions (“Don’t cut, rock outside, bad camber, etc). Beyond this, the driver has to negotiate the turn at speed using his best visual, auditory, and physical judgment of the road surface and car. To give more room for late corrections in the corner, rally drivers typically follow what’s called a late-apex racing line.
Many racing texts and references will show the “ideal” racing line as a smooth, symmetric line following roughly the longest path the driver can take from the outside of the turn’s entrance, turning in to almost kiss the apex, then smoothly rolling out to the opposite side of the turn’s exit. In the late-apex racing line, the driver rotates the car early in the entrance, giving a very gentle curvature to the path taken past the apex and through the exit of the turn.
There’s a number of advantages to using the late-apex line, but I think the easiest point to grasp is thatlate-apexing buys the driver time. In other words, if you are going really fast and you didn’t turn enough, then there’s not much you can do to fix the situation. However, if you are going really fast and you turn the car too much, there’s actually a few options for reliable recovery, so keeping the car on the road becomes a possibility even when mistakes are made.
The faster rally drivers can read corners instinctively with more accuracy than novices. They use this ability to make their decisions about how much to rotate and slow the car later in the turn which saves time and allows the car to carry more speed.
Aside from the racing line, drivers in rally also need to have a good mechanical understanding of the vehicles they drive. There is a high likelihood of damage to the car in a rally race, and it’s important for the driver to be able to get the car to the staging areas without tow assistance whenever possible to avoid time penalties but to also know how to limit damage to the car because there’s a fixed time allotted for repairs between stages.
The possibility of damage during the race influences the rally driving techniques directly. For example, many rally cars have limiters on the steering that prevent the driver from turning the wheels past a point where damage would be likely if the front wheels were to hit a severe bump. As a result, it’s very difficult to turn rally cars through a very tight turn radius at slow speeds. This is where the handbrake turn becomes essential – to swing the back end of the car around a slow turn. The handbrake technique is rarely needed for higher speed corners because the rotation can be created through weight-shifting techniques like trail braking or pendulum-ing.
As another example, rally courses can contain large bumps or ruts that could damage the car if driven over improperly. Rally drivers manage their steering angle over bumps and use weight-shifting techniques to protect the suspension on their cars.