There are two SI joints in your pelvis that connect the sacrum (tailbone) and the ilium (large pelvic bone). The SI joints connect your spine to the pelvis, and thus, to the entire lower half of your skeleton.
Like any other joints, there is articular cartilage on both sides of the SI joint surfaces. But unlike most other joints, the SI joints are covered by two different kinds of cartilage. The articular surfaces have both hyaline (glassy, slick) and fibrocartilage (spongy) surfaces that rub against each other. The joints also have many large ridges (bumps) and depressions (dips in the surface that fit together like a puzzle).
The Si joints are also unique in that they are not designed for much motion. It is common for the SI joint to become stiff and actually "lock" as people age. The SI joint only moves about two to four millimeters during weight bearing and forward flexion. This small amount of motion occurring in the joint is described as a "gliding" type of motion. Due to the small amount of movement and the complexity, finding out about the SI joints' motion is very difficult during a physical exam.
The SI joints are viscoelastic joints, meaning that the major movement comes from giving or stretching. This motion is quite different than the hinge motion of your knee or the ball and socket motion of your hip. The main function of the SI joints is to provide shock absorption for your spine through stretching in various directions. The SI joints may also provide a "self-locking" mechanism that helps you to walk. The joints lock on one side as weight is transferred from one leg to the other.