Cartilage - a structural connective tissue found in joints (and many other areas) which mainly consists of type II collagen fibres and proteoglycans.

Collagen - a protein which can aggregate to form fibrils, the structural unit of connective tissues found in bones (and many other areas e.g. the skin and blood vessels).

Long bone - one of six types of bone which is typically longer than it is wide and shows the characteristic "zones" (diaphysis, metaphysis and epiphysis).

Marrow - bone marrow is the tissue found in the interior cavities of bones which is the site of red blood cell production.

Matrix - a dense substance consisting mainly of calcium salts, especially calcium phosphate (hydroxyapatite).

Osteoblast - a cell that secretes bone matrix.

Osteoclast - a macrophage-like cell that erodes bone.

Osteocyte - a "mature" osteoblast which can no longer divide and is fixed in bone matrix.

Osteogenesis - the process of bone formation.

Osteoid - unmineralised bone matrix secreted by osteoblasts.  Its main constituent is type I collagen.

Osteoprogenitor cell - a cell which can, under influence of Bone Morphogenic Protein, differentiate into an osteoblast and start to lay down bone matrix.
Wnt pathway - a complex signalling pathway which, in terms of bone formation, seems to initially control differentiation of cells into ostetoblasts, and then later on, the balanace between bone matrix deposition and degredation.