Final answer:
Irregular bones, such as the vertebrae, have complex shapes and do not fit into other bone classifications. They provide support and protection for different parts of the body.
Step-by-step explanation:
An irregular bone is one that does not have any easily characterized shape and therefore does not fit any other classification. These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it from compressive forces. Many facial bones, particularly the ones containing sinuses, are classified as irregular bones.