What are osteoblasts?

The term means bone builder. . Osteo- always means bone. -blast means to build (or germinate).

These are one of three types of bone cells: Osteoclasts, osteoblasts and osteocytes. The suffix -clast means to break.

You may remember the term iconoclast which means the breaker of icons. But it is used today to mean some one who doesn't go the way others go. A sort of rebel or someone who questions and asks "why?".

Osteocyte just means bone cell. An osteocyte is found cell in mature bone and has an average half life of 25 years.

Osteoblasts originate in the bone marrow and contribute to the production of new bone. These cells build up the matrix of the bone structure and also play a role in the mineralization of the bone matrix.
Bone is constantly being built up and broken down by the body, making osteoblasts rather critical. The counterpart to the osteoblast is the osteoclast, a cell which is responsible for breaking down bone.
(WiseGeek)

As people age this balance tips toward the osteoclast side and people will have thinning of bone material (osteopenia and osteoporosis) making falls more dangerous.

The osteoblasts are critical to the structure and integrity of the bone. They don't just build new bone, they also maintain and strengthen existing bone, ensuring that the matrix is not compromised and that it is as even as possible.
(WiseGeek)