Leaf spring suspension has been around for a long time. Today they're found mostly on trucks and SUV's. They're located between the axle and the vehicle's frame and often accompanied by control arms and traction bars to prevent up and down and side to side movement.
Multi-leaf leaf springs contain several leafs stacked on top of one another. They have plastics and other materials between the blades to prevent noise. As they wear, the leaf spring may begin the squeak and creak with movement or weaken and begin to sag requiring curb height measurements. They are used in heavy applications instead of coil springs because the weight is better distributed across the chassis.
While inspecting the springs, look for any cracks in the spring's leaves. Leaf springs are made of steel and have a swivel shackle located at the rear of the spring that allows for the spring to elongate if the vehicle carries additional weight or goes over bumps in the road.
Removing and replacing the springs can upset the vehicle's driveline angles. A crooked axle and out of specification thrust angle causes the steering wheel to go off-center and the vehicle to dog track as it travels down the road.