Parallax Occlusion Mapping attempts to do what earlier methods like Bump Mapping could not. It raises a texture above the surface geometry to represent more complex curvature.
Typically, the graphics processor plays with vertices, and thus processing currency is often discussed in the number of vertices a given scene can handle. Around the development of Doom 3 (now iD Tech 4), a means of "baking high-poly models onto low-poly models" was developed, and that's sort of where Parallax Mapping comes from.
Parallax Occlusion Mapping is a pixel shader technique that uses the eye/camera vector to determine if pixels should be seen. Whether an object is seen or not depends on the computed value of the height map. The result is a 'raised surface' look you see on these rocks.
Unfortunately, Parallax Occlusion Mapping does not work perfectly at oblique angles, so you start to see some texcoord strecting.