The word osteon comes from the Greek word meaning bone. The osteon is the primary structure of the hard outer layer of bones known as cortical bone. Cortical bone, or compact bone, is dense, mature bone found in vertebrates such as mammals, amphibians, reptiles, and birds. It is this dense hard layer of bone which provides strength in the long bones of vertebrates; approximately 80 percent of vertebrate skeletons is composed of cortical bone. The osteon structure is comprised of the Haversian canal, Volkmann's canals, osteocytes, and canaliculi.

Osteons are cylindrical in shape and run parallel to the long axis of the cortical bone. Each osteon contains a central canal, or Haversian canal, along with concentric layers of bone called interstitial lamellae. Lamellae are composed of bone matrix, collagen fibers, and mineral crystals. Interstitial lamellae are contained in the spaces between osteons and perform the important function of providing stability to the long bones. The Haversian canal in each osteon is surrounded with rings of lamellae.

Within the Haversian canal is a layer of endosteum, connective tissue which contains nerve fibers and blood vessels. The blood cells contained within the Haversian canal supply essential nutrients to the osteon itself. These systems of Haversian canals and lamellae are also called osteons — osteon is synonymous with Haversian system. Both Haversian canals and Haversian systems were named after 17th century English physician Clopton Havers. Havers is considered the first person to discover this central canal in an osteon, which was named in his honor. He discussed the structure of bone in his book Osteologia Nova.

Volkmann's canals are also components of osteons. They are situated perpendicular to the long axis of the bone. They connect the blood supply and nerves to the periosteum, which covers the surface of bones, to the Haversian canals within the osteons. Found within spaces called lacunae, osteocytes are another part of osteons. In addition to sensing strain or stress, osteocytes maintain the bone matrix found within the lamellae.

Another component of osteons are the canaliculi, very narrow canals which join the osteocytes together in order to facilitate the distribution of nutrients and the elimination of waste within the osteons. Canaliculi radiate outward from the center, like the spokes of a wheel. They connect the Haversian canal with other canaliculi.

Because osteons are found only in cortical bone, they are not present in trabecular bone. Trabecular bone is also called spongy bone or cancellous bone and is usually found at the ends of long bones. Cortical bone, containing osteons, can be found in long bones such as the femur or tibia. The trabecular bone is normally surrounded by cortical bone at the end of joints and the vertebrae. This layer of cortical bone also contains osteons.